Room size-how do I determine 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).