i would have to say between 20 to 24 inches, 1920x1080 or 1920x1200.
size is more determined by how far you will be sitting from the monitor. i use my 40" hdtv as my main monitor for gaming.
as for resolution, the additional vertical pixels are nice for making the image more squareish but it might not be worth the additional price.
I don't think there really is a "best" size/res as this is higly subjective.