I generally enjoy a wide variety of characters, and so I'd probably say large as opposed to small. However, it's definitely a lot harder to make all the characters interesting and unique when dealing with large casts-- often, some of the characters will be superfluous and unnecessary.
I find it much easier to write for a smaller cast, but a large cast can be more rewarding in the long run, especially if you don't have to feature all the characters all the time.
EDIT:
Just to clarify, when I say I prefer large casts, I mean shows with more then maybe 6 characters. I don't mean to say that I prefer shows like the Simpsons where they have dozens of minor characters. Although they did handle that really well in the classic episodes.