Choosing the right bore size for a cylinder requires knowledge of your engine's standard (STD) bore size and a measurement of the existing bore using either an inside micrometer or both a telescope gauge and a micrometer. Using these tools, measure the cylinder's inside diameter around 5 millimeters from the top. Comparing the STD and the new measurement should suggest the right bore size.
The STD bore measurement for your engine and your own new measurements will lead to a suggested rather than a guaranteed number. The engine operates at such intense heat that the usually aluminum cylinders have a tendency to expand with constant use. In addition, excess material will collect in a cylinder over time, resulting in a bore that has shrunk by a discernible amount. That excess material should be cleaned out before a true bore size can be determined.
To be certain of the desired bore size of your cylinders, you'd need to machine your (presumably damaged or well-used) cylinder. Often the bore/piston size suggested by an initial measurement will result in a replacement piston too large or small for the cylinder, due to inexperience with the measuring tools, an imperfect cleaning of the cylinder or combination of the two. As such, precision in measurements and a clean cylinder are recommended before deciding on the size of your new piston.