I have the same bike. There is a stamp on your cylinder, a I or II, indicating its size. On my bike the stamp is near the area which houses the timing chain tensioner.
The original pistons are manufactured by Mahle not Vertex.
Let me know how much you paid and where you bought your kit. Mine has over 10k miles on it now and is due for a replacement. When I originally put rings in it at 3k miles the piston was near the high limit for clearance.