How do you found out what size piston you need for your 1993 kx 250 The numbers on top are 617M06640?

That is a stock bore 93-01 KX250 Piston. The part number 617M06640 is a Wiseco number. I know as I have the same bike and this is the piston kit I used. Very good piston. Do not be suckered into a single ring piston as it will not last as long.