Start with a 600. That's what I started on and it worked out well for me. The hardest part is just "getting comfortable". Be cautious, but be confident. If you think you will drop, you probably will. Learn your positioning, slowly progress your speeds, get used to the release points on the clutch, and feel out leaning capabilities according to speed. Just don't ride beyond your ability- there is where everyone goes down- just trying to keep up with soemone else. Just remember your on 2 wheels.