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

You're getting some good advice and some terrible advice, which is what happens when one shouts out a question in the mall parking lot that is Audiogon.

The square room, even with the 6 ft. vent, creates a special problem because of the extreme room modes it is probably creating (exact effects depend upon furnishings, room construction, and other factors). Simply put, it will cause some bass frequencies to be exaggerated and others to be deemphasized. To address this, you will need a speaker with adjustable, powered bass, or monitor speakers augmented by a subwoofer, along with a lot of patience to try various set-ups in your room.

The room is also small, which rules out many midsize and all large speakers. Putting a large speaker in a small room that is also square is a recipe for absurdity (Salon 2's, which have prodigious bass, in a room that measures 11' x 11' x 8' would be an terrible move, as would Studio 2's).

One good, reasonably sized speaker with adjustible, powered bass is the Vandersteen Quatro (there are of course others). There are a lot of good monitors and subs out there. If you like the Revel sound, Gems and a Revel B15 would work and make you happy.

The other posters are way too polite - big speakers sound like shit in too-small rooms. I will, however, add that there is a big difference between Salon 2's and the other speakers you mentioned. Salons have prodigious bass capabilities - while they do not have the spl limits and extension in the bass of the physically larger originals Salons (I owned that speaker for three years), they still have bass that few speakers can equal. The Cremonas and Reference 3A Grand Veenas, on the other hand, are more in the nature of midsize speakers, but with dual woofers, so again, they will be a lot for a room of that size.

More importantly, you have a square room - you want a speaker with adjustable bass or monitors with a subwoofer.

Sorry to go off-topic, but in light of the last post, I am compelled to comment about the Absolute Sound. Buyer beware - since resurrected a few years ago, its reviews have become overwhelmingly positive, and positive in a uniformly over-the-top way. Take any two or three consecutive issues and read the reviews - pay no attention to that man behind the curtain.