Will he be bored?
Although cats are not really the "loners" some people believe them to be, a cat can remain active and interested in his surroundings, no matter how small, as long as his human companion takes the time each day to play with him and cuddle with him. After all, this is the reason you're thinking of getting a cat, isn't it?
Will litter box odor be a problem?
Litter box odor is controlled by two conditions:
- Choice of Cat Food (Garbage in: Garbage Out) - As a rule, premium cat food with few grain fillers will produce less stool odor.
- Cleanliness of the Litter Box - A box kept scrupulously clean will not produce penetrating odors, even if it is kept in a small bathroom. (If you're really cramped for space, there are some very nice litter box enclosures on the market, that look like nice pieces of furniture to visitors.)
I don't have a yard. Will he be unhappy kept indoors all the time?
Please don't fall for the "fresh air and sunshine" fallacy. Hundreds of thousands of cats live out long and perfectly happy lives as indoors-only cats, as long as they get the exercise they need through interactive play with a human parent. There are safe alternatives, depending on your apartment arrangement. You can screen off a balcony (with the landlord's permission), or you can train your cat to walk on a leash, for outdoor excursions.
The short answer to the original question is really, "The amount of love and positive attention your cat receives is much more important than the square footage of your apartment."