The most commonly used breeds (and mixes) for service work are about 60 pounds in size. This is the size that has been shown to be the most successful in the largest number of applications or situations. However, there is no restriction as to whether a given dog might be a good service dog based only on size or breed. What matters is how effectively and reasonably he can execute his duties.
It is very unfortunate for owners of small breed service dogs that so many pet owners choose to pass off their small breed pets as service dogs. Unfortunately, when a dog is small enough to carry, it is far easier to disguise that dog's lack of training than when the dog has to walk on his own four paws and act like he knows what he's doing and this makes small dogs particularly appealing to fakers.
A person with a small, large, or otherwise non-traditional breed dog being used for service work should anticipate encountering greater resistance and conflict in public over their dog even when it is a legitimate service dog and not a pet. Do not make such a choice and then bemoan the extra challenges after the fact. Instead, know ahead of time what you may be getting yourself into and choose wisely.
Know what you are really getting into when you consider a small, large, or otherwise non-traditional dog for service work. The chooser is responsible for the consequences of the choice, not the public. If you choose a dog that is not capable of walking long distances, it is not the public's responsibility to provide you a way to compensate for your choice's lacking. If you choose a dog that does not fit in the space provided it is not the public's responsibility to provide you with additional space. If you choose a dog that is more likely to be injured or is more easily injured, it is not the public's responsibility to compensate for your choice--it is yours.
So know this about the special risks of handling a small service dog.
1. They typically can't walk as far because their legs are shorter, making them work harder to go the same distance as a dog with longer legs.
2. They are typically more fragile than dogs with stouter bones.
3. They are typically shorter than more traditional service dogs.
Know this about the special risks of handling a large service dog.
1. He may not fit where you want to go.
2. He may come face-to-face unexpectedly with children or people who are seated.
3. He may tower above food that others intend to eat.
None of this entitles you to special extra accommodations from the public, not when you had the option to choose a dog that did not have these extra encumbrances. It's really no different than choosing a wheel chair that is missing a wheel and then expecting the public to help you overcome the difficulties of the missing wheel when you could simply have gotten a chair with all the necessary parts to start with. Your choice means your consequences--no one else's.
If you choose to carry your service dog because he is tired or because you are afraid for his safety, that is your choice. The public are not responsible for making additional accommodations for your dog's impairments, just for your own. That means that if you carry him, it needs to be in a way that your service dog does not impact the public more substantially than an ordinary service dog would. Here is a simple guideline that apply to all service dogs, regardless of size:
For the sake of clarity and brevity, we will call any fresh food displays, ready-to-eat foods, and surfaces, dishes or utensils upon which prepared food is served simply "food." We are not including food that is packaged for transportation to another location such as shrink wrapped or boxed items not requiring refrigeration. We are also not addressing what you do with food you consume yourself privately. Whether a stray dog hair in your food is disturbing is a choice you can make for yourself, but not for others.
However high your dog is above the ground, that is how far he should be from "food." So if your large dog is three feet tall (at the highest point), then he should be at least three feet away from any surface that contains "food" (such as a restaurant table or fresh meat display in a grocery store) that is three feet high or lower. If you are carrying your dog and that puts him four feet above the ground, then he counts the same as a four foot tall dog in this equation.
If you need to be nearer "food" than the height of your dog allows, then lower your dog below the level of the food. If your dog is tall and you wish to eat at a restaurant table, put your dog on a down under the table. If you are carrying your dog, also put him under the table.