Smaller dogs have traditionally been used for hearing alerts. Recently, more people are choosing small dogs and toy breeds as their service dog for other disabilities. Owner-trainers are selecting them for diabetes, seizures, PTSD and Anxiety. The benefits are easy to observe but examining the concerns are also worthwhile to make sure dogs of the smaller size are actually a good fit for the handler, the family environment and their resources.

Benefits 
A dog's small size means they may:

• be cheaper to feed
• have less fur overall (but still have grooming requirements)
• be easier to transport
• be easier to tuck out of the way
• may need less exercise than a larger dog (but not always true)

Concerns

• have different health issues as a group than larger dogs
  patellar luxation (knee cap) 
  protruding eye balls (especially in short-nosed breeds)
  hypoglycemia (small size/fast metabolism means they have to eat more frequently to maintain normal blood sugar levels)
  tracheal collapse (means you will need to use a flat walking harness)
  Legg Calve Perthes (hip joint issue)
  chronic valvular disease  (heart disease)
• may have higher incidence of cryptorchoidism (undescended testicles) than larger dogs
• anal gland issues are more common in small dogs
• tooth and jaw issues are standard among small dogs (which also means more dental care, and smaller food which is more expensive)
• may be harder to potty train as they can sneak through small holes to potty out of sight in the house or may not be able to hold their bladder as long as larger dogs
• small dogs tend to be over-represented in puppy mills. Rescues/shelters take in many puppy mill dogs. These are dogs with unknown genetic, medical, and behavioral histories and do not make good service dog candidates. 
• tiny dogs are not likely to be as effective in performing physical interruption type tasks
• may not be able to retrieve/drag larger objects
• may not be able to access higher locations/steps without help
• terriers like Jack Russel and fox terriers may need more exercise than you think!
• terrier breeds can be very persistent and predatory (including the tiny Yorkshire terriers)
• do not adapt well to harsh environments -may get cold or hot quickly in harsh environments or on hard floors
• shiver more often (draws attention to your dog, may need a coat in indoor environments)
• vet bills cost the same for small dogs as medium dogs. Sometimes spaying/neutering and operations can cost more due to the skill/attention to detail needed for operating on smaller bodies. Dental surgery is expensive as it requires a specialist.
• fragile structure-falling, jumping or being dropped from even low heights can break bones
• may be too environmentally sensitive or over-reactive-smaller dogs have have a faster metabolism, their flicker fusion rate in the eyes of small dog are higher so they tend to see more motion than larger dogs, tend to move faster, be more fearful
• may be more prone to alarm barking (unwanted as a service dog and you can be asked to leave if you cannot control your dog)
• most small dogs do not tolerate or enjoy being handled by children 
• not as easy to socialize with other dogs and animals due to size difference and predatory issues
• may be injured if children are handling the dog (stay with medium and larger dogs with more solid structure and temperament if the dog is intended to be a child's assistance dog)
• ears harder to clean due to size (make sure you have the dexterity to do so or can hire a groomer regularly)
• may trigger predatory behavior in larger dogs you encounter in public
• may get stepped on (and have to be carried more often as a result, you will need to bend over to pick up a small dog)
• may not be taken seriously by retailers or accommodation providers (may be mistaken for "fake" service dogs (dubious about effectiveness of small size, unfamiliar with your breed as a service dog, etc)
• may attract unwanted attention from public
• you will be bending over for the lifetime of the dog (to reward behaviors, do hand targets-sue a stick, lift it over high barriers, keep him from harm etc)
• you will be sitting or kneeling to train at times, or elevating the dog for training

Tips:

• Avoid breeds that have been "bred down" from a larger standard
• Avoid the toy breeds (dogs smaller than 15 lbs)
• Choose lines that have a heavier (more sturdy) bone structure
• Choose a breeder than breeds on the large size of the standard or get a mix with a slightly larger (also suitable) breed
• Find out what health tests have been done on the dog
• Find out about the genetic history of teeth of at least 3 generations back
• Brush your dogs teeth daily and give him things to chew
• Have regular dental check ups
• feed adult dogs at least twice a day, carry extra food for long days
• Watch for irregularities in gait, like a skip off one leg or the other now and then when running (patella)
• Avoid putting your dog in a shopping cart, use a snuggle/huggie tyoe carrier instead if you must keep him off the floor
• teach him to be confident on his own and where to tuck himself out of the way to avoid injury

Small Breeds to Consider

• conformation line beagle (breed only for companionship for many generations) (avoid hunting lines as they are higher energy, high prey drive and nose -oriented)
• conformation bichon frise
• Moyen poodle
• Miniature poodle (avoid toy sized)
• and mixes with the above breeds in them

Carefully consider your disabilities, the tasks the dog will be performing for you, your lifestyle, exercise levels, personality and those living around you (family and caregivers and other members of your support team), costs and make sure that the individual dog you choose is right for you. 

If you have a disability and are considering a service dog to help you mitigate that disability, here are some things to consider: (whether you self-train or get a dog from a program)

Is a Service Dog the Best way to Mitigate your Disability?
Make a list of your impairments, the things you need help with. Are there other non-dog ways that can help you? 
What are your biggest challenges? 
There are more and more assistive technology created today that is effective, undetectable and more cost-effective than a living breathing being.

• More portable
• More reliable
• Less intrusive

A dog may not always be available to help you. 

Is it cost-effective? One time costs rather than ongoing costs of a dog (feeding, vet, grooming, training, maintenance etc)

• Having a service dog with you in public is stressful
• draws unwanted attention
• accessibility challenges
• emotional toll of failure

While the presence of a dog can help you feel safer, they cannot be protected trained or pose any threat to a member of the public. When you need help, a first responder will need to approach and touch you. A dog needs to be very comfortable with that. Especially if you are unresponsive and cannot direct your dog. 

Considering Others Needs 
To be fair, having a dog in public with you does affect others. Just as they need to respect you and your needs, you need to respect their needs. Whether it's fear of dogs, allergies or the effect of a dog on the health of other animals, the handler needs to think about the team's presence and impact on other people and animals.

Ability to care for a dog on a daily basis-getting outdoors for 4 X or more a day to potty, one or two exercise walks or training outdoors

Ability to go out into public for acclimation, training and public access work?

Dogs make mistakes in public.

The hander needs to learn to speak dog.
Dogs communicate mostly with body language. To be a successful part of a team, you will learn your specific dog's dialect of dog. His needs will need to be met just as your are. 

Being part of a team. Trust your dog that means giving up some control. 
It also means giving up privacy. Your dog will spend the vast majority of his life within 6 feet of you. If you enjoy your space, this will feel invasive. 

Carrying equipment around. When you travel, it's not just the dog that comes along, but also the equipment a dog needs to function. It's like travelling with a toddler. There is extra gear to bring with you. 

With this, spontaneity disappears. Spontaneous people don't do well with service dogs. Unless they are organized. Organization skills and a spontaneous personality don't generally go together. LOL! 

Slows you down. Because of the above. Because part of a 2 part team etc. 

Must Pick up poop. Even blind SD handlers must pick up their dog's poop in public places.

Not easily accepted in work places or schools. 

Access is limited only to places where the public can go. Under the ADA, work places, churches and private schools and universities may have their own rules and you need to get special permission to take them there. Some of these places will not want a dog there for many reasons. Other places like food preparation areas and operating theatres service dogs are not allowed as they are not open to the public. In some jurisdictions such as Ontario, Canada, dogs may be allowed in non-public places. Check your local laws as they apply.

You will have to learn a series of laws: federal, state or provincial and even city or municipal as they apply to service dogs. 

Having a service dog adds to the familiar work load. While a service dog may solve one problem, the dog may pose others that the family doesn't want to deal with. If you have an already busy life, adding a service dog (either in training or a program-trained one can push you over the edge of what you can handle. I't more like adding a chid to your life than adopting a dog from a shelter. There is so much more involved in living with a service dog. Even more if you are training your own.

Do you have the space for a dog? Fenced yard or other safe space the dog can exercise off leash. Stairs or elevators to the outside can pose a problem, especially as a puppy or if your dog develops intestinal or badder issues.

Invisible disabilities and a service open up the questions of who you are training the dog for and the public can get personal very fast, watching to know the details of your condition. Do you have the ability or desire to politely rebuff or redirect them from personal discussion? 

How is your life going to change in the next 5-8 years? Will a service dog still fit in it? Are you planning a move? 

Can you set up your own support system? It takes a community to raise a service dog, from selecting to raising and training and maintaining the dog's every day needs and ongoing training for life. Emergencies arise for both you and the dog and he still needs to be cared for.

You may need to fundraise to help pay for the costs of training the dog. Most programs require that you put sweat equity into the dog and owner-trainers are responsible for all costs of the dog. 

Assertive to have your needs met without impinging on the needs of others.

Tolerate being ignored- focus is on your dog and people ignore your communications even if you step between the and your dog to physically block them. 

Do you have other dogs at home that are not dog-friendly? Your new dog may learn bad habits from that dog or even get hurt by your current dog. Because dogs are social learners, when they live in a dysfunctional social environment, they learn unwanted behaviours from each other. This is often despite much training. Social learning can be more powerful than other kinds of learning.