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It's not the size of the dog that causes allergies. Poodles come in all three sizes (standard, miniature and toy) and are considered one of the least allergen producing breeds.

The others are correct that your landlord cannot restrict the size of a real working service dog.

A small dog is not going to be able to open doors. He just won't have the weight and strength to get the knob to turn and you'd have to put extenders on all the doors and he couldn't do doors outside your home because of no extenders, and so on. So if you need doors opened, you're going to need at least a medium sized dog.

Retrieving is also problematic. Small dogs cannot retrieve large objects. They can't handle the weight and their mouths don't open as wide. It's also very hard on a small dog to ask it to repeatedly jump down, get something, then jump back up again over and over throughout the day. These little dogs are jumping from and to heights considerably over their heads and it's very hard on their joints. So again, if you really need a dog to retrieve things for you, then you need a medium sized dog.

Very few programs are going to place a small dog for all of the reasons I've outlined above, so you'd have difficulty finding one anyway. Unless you are already a skilled dog trainer, in which case you wouldn't be asking this question, you really wouldn't have much chance of successfully training one on your own.

So. You have a choice. If you need an actual service dog, start applying at programs. The waiting period is typically a few years, which is not surprising since it typically takes 18-24 months to actually train a service dog. The program will pick the dog that suits your needs and you'll probably have little choice on breed, if any.

Or....if you don't actually need the dog to perform tasks for you, you could get an emotional support dog instead for the depression. No special training is needed beyond ordinary good pet manners. You can pick one out at the pound. However, be aware that an emotional support animal is a pet, not a service dog, and it cannot be taken places where other pets are not permitted. Your landlord would also be able to place a size restriction on an emotional support animal.

If you want a service dog, here's an article that explains how to get one, including instructions for finding a program to apply to: http://www.servicedogcentral.org/content/node/262