Highest overall seat height on a sport bike is around 33 inches on Yamahas I believe. At 6'4 you should be able to put both feet flat on the ground, which is a good thing. At 180lbs, sag shouldn't be much of a concern either. For a more upright ride check out Suzuki's DRZ 400. These bikes are...
Definitely go with a used 600cc bike to start. 1000cc's is not necessary unless you weigh more than 250lbs I would say, otherwise you're asking for trouble with that much power. I'm not personally a fan of GSXR's just because everybody has them and they are the most common bike to be stolen...