Lots of great advice by others, so let me just add that I'm 6'0" and 175 lbs, and I fit just fine on my CB550, not cramped at all, very comfortable riding position, the CB550 feels bigger when riding it, than it looks when parked. The CB550 is very nimble, making it very easy to ride slowly, which is great in town and in tight turns, but with experience the CB750 is too. I would look for a CB750 if I planned to *regularly* take long distance trips or ride tow-up, otherwise the CB550 is capable of both.
Another thought to consider, is how comfortable are you with higher engine revs? The CB550 really likes the 4-7k range, which for some ridiers can seem too high, in which case the CB750 might be more comfortable to ride. I like the higher revs, so the CB550 suits me very well.