piston size?

gotta get new pistons, not sure how to tell if mine are stock bore or not. the only markings on top of the piston is a 15, but i've searched quite a bit and cant find any information on what that means. i plan to have the engine bored, but obviously only if there is a piston available bigger than what i have in there. any info helps.
2003.5 MSP
Wiseco makes stock size and over-bore pistons for the Protege. They require forged rods though due to the wrist pin style.

There are also oversize stock pistons available from Mazda or aftermarket parts places (ex. rockauto)
2002 Mazda Protege
I would invest in tools to measure the bore if you are doing some internal engine work. Wiseco pistons are popular and available oversize.