Do you want to get it rebored?
Get the new piston first and let the engineering workshop bore the barrel to fit your specific piston.
If you only want to determine size and eventual wear get them to measure the piston and barrel in relation to each other. Clearances are pretty much standard and depend only on diameter and material of barrel (cast iron or aluminium coated)
If you measure yourself note that only barrels are supposed to be cylinder shaped. Pistons are often tapered and oval as they are machined at or near operating temperature whilst you measure at room temperature.