My personal opinion, buy what you want and what you will be comfortable on and learn to contain yourself till your abilities are up to what the bike can do. I totally under stand where every one is coming from with start slow and small but if your not excited about what you have whats the point. Just because a 600rr might have 110HP dose not mean you have to use it, that's where you have to learn to control yourself. I learned on a Gsxr750 and i did fine, one of my friends bought a busa as his 1st and is a great rider and never been down. you just have to be smart about what ever you choose, if you have no self control go small, but you will quickly grow out of it and want a upgrade. Like i said jusy my opinion. If you was a recomendation from me; based on your size and assuming your wanting a sport bike i'd say the SV650 or a ninja 650r or the fz6r, decent sized bikes with mid to moderate range(no aggressive) power, that you wont out grow for several years.