how big a speaker can you put in an 11 x 11 room?

I agree the main issue is how far you can get from the walls. I have a small room myself, and one set of rear-ported speakers (Soliloquy 5.3i) clearly overwhelmed it. A much larger rear-ported speaker does a lot better! (Usher CP 6311). So I guess it's the speaker design too. Go for front ported if you can

Great point Dwhitt - Arros are great, especially with Jazz on vinyl, the OP's stated affliction of choice. Would also throw in a suggestion for one of the Kef monitors - I think the XQ series unless you plan to spend way more for the much better Reference. I owned some of their older low end ones and they were pretty amazing for the price in small rooms. Another to consider, is Usher series, they tend to throw a great soundstage in cramped quarters, not an easy feat.