"What size head do I need?"

Think big then turn down the volume. A small amp driven hard tends to sound rougher than a big amp taking it easy. Also a big amp gives the bass end more depth due to the ability to handle the peaks. Using Yamaha CP2000 at present; lotsa guts, not too heavy, cheapish here in the UK for such a powerful amp (420 pounds sterling mail order).

"Stereo or Mono"

I run in stereo, although the vocal echo return is in mono despite use of a stereo effects unit; use of the stereo return seemed to "defocus" the vocals. Strings, electric pianos and all things chorused sound much better stereo.

Of course, given the nature of audiences as discussed elsewhere on this forum, the only person who notices the difference is you!