When to go up in size

[Longbow62]

I pulled the heads and cylinders off my project bike. The pistons showed no damage, and the cylinders looked fine. One of pistons is marked 64mm. What cylinder measurement determines boring and
moving up in piston size? One head had way more very hard carbon in the combustion chamber than the other. Also around the combustion chambers shows uneven wear. If I drag a nail around where the chamber was cut the shallow lip kinda disappears like it was worn away.     

[1976RD400C]

You really need a bore gauge to measure your cylinders to check for wear, taper, and out of roundness to determine if they need a rebore to the next size. I would take a sheet of fine sandpaper on a flat table and resurface the heads and check how flat the gasket surface is. The difference in carbon build up is no biggie.