Generally, 2 watts will run any model of π speakers and get pretty loud. I wouldn't say they're blaring loud, but they'll surprise you with what they can do with two watts.

As for room size, you have a lot of good choices there too. All but the largest speakers work well in small to medium size rooms. I wouldn't put a seven π-18 in a small room, but I've had seven π cornerhorns with 15 inch woofers in 12'x14' rooms and they sounded very nice. That's a pretty small room for speakers like those, but as long as you sit more than a couple feet away, they sound good.

On the other end of the scale, I like the little one π, two π and two π tower speakers for small rooms. They don't take much space and sound great. If you have a little more room, you might consider the three π or eight π speakers.

By the time you get to the four π, six π and seven π speakers, you're starting to take up a lot of real estate but like I said, they sound pretty good if size isn't a problem and you can sit more than a couple feet away.