What is the ideal listening room size?

What would be the ideal dimensions for a listening room? Is the absolute size important, or is it mostly the shape?

Also, what floor, ceiling and wall treatments, or is just plain-old sheetrock acceptable? Budget for this sort of stuff would be very low, so economic suggestions appreciated.

I realize there may be other threads on this, but I wasn't sure what to search on. Also, I'm no acoustical engineer (can barely spell it), and relatively new to this hobby, so the simpler and more basic the suggestions and reasons, the more appreciated! (And no noob flames, please!)

Thanks!