Well, 5.5" x 5.5" and 6.5" x 5" are the clear choices. Esthetically I would say 6.5" x 5" has the better proportion visually, and that was my vote. Functionally, I would probably prefer to have 5.5" x 5.5" though. Five and a half inches is a perfectly respectable dick length, long enough to screw with in a variety of positions and 5.5" is an above average girth. But the above average girth would make the average length seem rather short.
Still, some folks seem to like that type of appearance, and many of them seem to be female.