Agree with Onemug, size depends on how loud you like to play them and how close you sit. In general, if you like to play the loudest orchestral music at levels approaching natural ones, e.g., 115 dB on the peaks (which is maybe 100 dB on the typical SPL meter), you will either need a big amp, or to accept some clipping. But many or even most people never listen that loud. If you back off 10 dB, you've reduced power requirements by a factor of 10, but perceived loudness by only 1/2. So the only real rule on size is to get what's right for you.
