All I can really tell you is absolutely do not go over 600. I've ridden dirtbikes all my life and I could barely handle a 750/1k when I rode one and I'm 5'11 175. Not much of a size difference I guess, but it plays a big part on a motorcycle. Personally I am not going to opt for a 250 for my first bike, but if you don't have previous experience with a motorcycle then a 250 Ninja as some said would serve you well as a starting point.