I live in a 534 sq/ft house, with my husband, five cats and three dogs. I have two rotweilers and a boston terrier. We raised Frau from a puppy in an apartment (which was actually bigger than the house we have now!
) on the third story. We didn't let her onto the balcony unless we were with her, and the manager said it was actually illegal to have a dog out there without a human. And to have them potty out there.
My advice is to research your breeds. Decide what you want/need in a dog, go to dog shows and talk to breeders and handlers, pet the dogs, see them in action. Watch the obedience trials. Even if you decide to rescue, knowing what breeds went into your mix will help you understand their drives and training requirements. My brother has a bulldo/JRT mix. She looked like a bulldog as a puppy, so he thought she would be mellow and big. Instead, she is a 40 lb jack russel with an unlimited supply of frantic energy and enough drive for ten dogs. Certainly NOT the dog he should have gotten.
I personally don't feel that the size of the dog limits what breed/mix can live in an apartment. As long as they are properly and lovingly trained and exercised accordingly, any dog can live happily with it's owner.