Room size-how do I determine speaker size?

HI. I am moving soon (to a small house), with a living room of 12x12. How do I determine what size of sperkers will work in the room? Monitor, 2-way floorstanding, 3-way floorstanding? I was thinking that monitors may be a good choice, but I don't want to totally miss out on bass response. Is there a "rule-of-thumb" regarding room size and speaker size?
Generally speaking you couldn't have picked a more difficult size room to work with. You will have some suck outs and boomyness. Monitors usually work best because they are easier to place than floorstanders and don't excite the low end as much. Once you get your monitors set up to their best effect, you MIGHT get some mileage out of a sub. I doubt it but its worth trying.
I would have to agree with the monitor suggestion. The general rule of thumb is small room-small speaker...

The good news is small speakers are generally cheaper than larger models. Just don't rush into anything.
I don't want to totally miss out on bass response
You can't get direct low frequency response in that room -- only reflected (the wave-length is longer).
So, I second, third & fourth the monitor. You'll have great sound and "apparent" bass response (at the expense of spkr sensitivity).
The common misconception is that in small rooms the low frequencies must build up in the room to be heard. What we hear is due to compressions and rarefactions in the air. As the cone of the speaker moves back and forth it creates these variations in air pressure that then travel through the air to your ear. If the cone moves back and forth at 20 Hz then the ear hears it as 20 Hz. The ear doesn't know how far these waves of pressure travel before they arrive. You can put your ear an inch away from the woofer and hear the bass.

Yes, the dimensions of the room will play a huge part in reinforcing certain frequencies, but you don't need a big room to get bass.

I agree that small rooms can be problematic for a variety of reasons, and one with equal walls even worse, but you can still get plenty of bass. I also agree with monitors, and I would use a sub. Most big speakers require that you sit further away than this room will allow.
I recently went through a lot of back and forth auditioning to satisfy similar concerns in my 11' x 16' room. Ultimately it was the coherence or clarity of my favorite monitor that won out over the 3-way tower. The 3 way was well mannered, and the bass rich without being overwhelming. But the subtly more direct, clean sound of the monitor won me over. It seemed like all the music could open up at lower volumes. I found its bass quite sufficient for my own tastes, although certainly not as deep and full.
This question is easy...The size of a speaker allowed for any given room is decided by the tolerance of your wife or significant other.

I usually start big and work my way down until the yelling stops.

While I generally agree with the monitor-and-maybe-sub advice as a starting point, I'd recommend that you experiment with different types of speakers in your price range. Some tower speakers use small drivers and aren't really that large in volume, so they produce less bass than you might think (as well as less bass than the spec sheet says, but that's almost universally true).