Plastigage is a wax thread that is precisely sized. You break off a 1/2" long piece and place it on a dry crank main or rod journal and bolt on the cap, with bearings, torque to spec, then take the cap off. The wax thread will be squashed out to a thin layer on the journal. Finally you take the paper envelope that the Plastigage comes in and hold it next to the wax streak. There is a calibrated scale on the edge of the envelope that corresponds to the bearing clearance. When the width of a dash on the envelope matches the width of the wax smear, yo read the clearance number printed on the envelope and compare that to the spec in your rebuild manual. Very accurate. You can get Plastigage at any parts store.
Your rebuild manual will list a spec for the piston/cylinder clearance. Shove a piston half way down the cylinder and insert larger feeler gage leaves in until there is resistance. Compare the thickness of the feeler gage to the spec in the book. If too tight, you must hone the bore until it meets minimum clearance spec. If too loose you need larger pistons. I have never had a problem here but if I did, I would take the whole mess back to my machinist and tell him to make it good at no charge to me. He would have screwed up.
Finally, fitting rings is the one thing the home mechanic must do himself. Place a ring in the cylinder bore and shove it down an inch or so with the top of an upside down piston so it is square with the bore. Measure the gap of the ring with a feeler gage and compare that reading with the spec in the manual. If it is too tight, you must file the end of the ring until it is large enough. If it is too big, take the rings back to your auto parts store and get a set for larger bore.