since it's only Y15, go 250cc first to learn the dynamics of a sportsbike

you can go straight to 600cc if you are familiar with riding a 250cc before, or 150cc two strokes like yamaha tzm, honda nsr etc. Or even 150-250cc four stroke bikes that resembles a sportsbike or naked style like suzuki fxr150 or yamaha fz150. You need to train your hips on gripping the tanks and get used to it before even trying a 600cc, because on 600cc you should concentrate on controlling the bike and not getting used to the new aggressive riding positions.