I think the "ideal" size depends on a number of factors. First, of course, is personal preference. Next would be the size of the base. The silicone stoppers, cork, or the smaller stainless steel base from Ruth Niles would probably all work with a smaller (in both diameter and height) stopper on top. The bottom heavy bases like the CSUSA droplet style or the PSI "vintage" might accommodate something a little larger on top. The basic cone shaped bases probably fall somewhere in between. Finally, a lot depends on the specifics of the shape you turn. I've done some, for example, that are turned sideways (think of a disk with the stopper base screwed into one edge) that are certainly wider (in one dimension) than other styles I've turned, but light enough to still offer good balance.
-Barry