Use a 3-4 inch micrometer to measure the piston 90 degrees away from the wrist pin and at about the same level. It will measure about 3.497" or a bit more for a standard bore, and .020", .030" or .040" or more larger if the block has been rebored. Most oversize pistons will have the size stamped on the top of the piston once you get the carbon scraped off.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!