I'm looking to get my block bored/honed soon and I'm shopping for pistons. Why are there 2 specifications for the size? I'm looking at srp pistons and it says 81.25mm (the bore size I plan on getting) and then it says +0.020 oversize. What does the oversize mean?

 

it refers to oversized compared to stock size, thats .20 over oem standared bore

 

If your stock pistons are 81.mm then 81.25 is fine.
+0.020 simply means 20 thousandths over stock bore.
81mm is OEM bore so .25mm or 0.010 thousandths of a inch is the area/meat you have to work with in the cylinder to do the clean up hone considering your piston wall setting.
If your cylinders are scored or out of round you may need to go to 0.5mm over.
Most piston manufacturers make pistons slightly over OEM / stock bore size for rebuild customers like yourself.
Depending on the type of material your cylinders are made of and final piston wall clearance you decide on as well as the application turbo or N/A of your build. All those things need to be taken into consideration before you purchase pistons.