Funny how this question comes up all the time, as it's similar to asking, "How big of an engine do I need in my car". You could probably get by with a 50 hp engine in a 3,000 lb car, but it'd take 2 miles just reach 70 mph and that would be it's limit. If you wanted any kind of thrill of driving you'd probably start looking at 200 hp engines and bigger.
Math time:
Speaker efficiency is rated by sound pressure (dB) output per watt input at one meter in front of the speaker. Typical speakers run about 85 - 92 dB/w/m, but can range from 80 to 109 dB/w/m. These ratings are based on having no room reflectance. Note that one dB is the smallest difference in sound pressure (loudness/volume) that the average person can perceive. Conversational sound pressure levels run around 60 dB, most audiophiles listen at roughly 85 dB, symphonic concert peaks are 105 dB, rock concert peaks run up to maybe 110 dB. Keep in mind that these are peaks. Sound pressures for rock doesn't vary nearly as much as other musical forms, especially classical.
Smaller rooms provide more reflectance than larger ones, partly because you're forced to sit closer to the speakers. Rough rule of thumb is that a 2,000 cubic foot room (say 12 ft x 20 ft x 8 ft) with two speakers will result in acheiving the rated speaker efficiency (room loss = gain from using two speakers). From your room description you may see a 1 or 2 dB net gain.
The relationship between speaker sound pressure output and power input is logrythmic. So a 3 dB gain in volume (sounds half again as loud) requires twice the power and a 10 dB gain (sounds twice as loud) requires ten times the power. Since speakers are rated at one watt input 1 watt = 0 dB of gain. Examples (using two speakers in a 2,000 cu. ft. room):
85 dB efficiency speakers with 100 wpc amp = 85 + 20 = 105 dB
90 dB efficiency speakers with 40 wpc amp = 90 + 16 = 106 dB
95 dB efficiency speakers with 10 wpc amp = 95 + 10 = 105 dB
If you're a rocker:
85 dB speakers need a 400 wpc (26 dB gain) amp
90 dB speakers need a 100 wpc (20 dB gain) amp
95 dB speakers need a 40 wpc (16 dB gain) amp
You can see why P.A. speakers must be highly efficient as bigger rooms would otherwise require even more power. IMO high quality stereo amps start at $500 for 2 wpc, but quickly get much more expensive with $2,000 for 150 wpc being common. Some of the new digital amp technology based receivers are the exception to the quality versus power versus cost paradigm ($200 can get 100 wpc for 5.1 systems).
Important: Speakers can handle excessive amounts of un-distorted power much easier than lesser amounts of distortion. This is because as amps are asked to exceed their rated output the nice smooth waveforms that music represents get flattened off at the peaks (this is called "clipping"). Not so bad for the amp, but the speaker is simply trying ot follow the signal fed to it by the amp and instead of smoothly following musical forms each driver is being asked to stop and start instantly. This results in energy being delivered and it has no place to go, so the extra energy is released as heat that melts the drivers voice coils.
But you say, I don't plan on listening at levels of 100 dB or more. Well, peaks "sneek" up on you pretty easily. And IMO you want to size the amp to be able to keep a very good grip on the speaker and bully them around a bit, similar to sizing the engine in a car. So I'd recommend sizing the amp to at least reach the peaks mentioned above. (Always check to may sure the speakers are rated to handle that much power.)
