how will i know what size piston i need?

if nothing else gets in the way i hopefully will get this bike rebuilt soon.anyway,when i pull the cylinder what do i have to do to figure out what size piston i will need?if i will need measuring equipment,what will i need and what kind of price range can i expect to pay?i dont have alot to spend so would it be cheaper to have it measured by a shop?anybody have any opinions on what piston i should go with?im looking at wiesco or vertex.my buddy that i bought the bike from said he always stuck with vertex which is whats in the bike now.just would like to maybe hear others opinions befor i decide.also if i will need it any resources of where to buy the measuring tools?thanks in advance for any input.oh yeah my 01 cr125 is the one getting the top end done.

thanks for your replys,when i pull the cylinder is there anything particular i should look for to know if it is acceptable to just replace the piston?i spoke with my friend who i bought the bike from awhile ago and he said something about that vertex makes pistons in slightly larger sizes to compensate for cylinder wear?if i order the standard size,is there a way to tell without measuring it if the tolerances are within spec?just didn't want to end up ordering the wrong size and have to go through the hassle of having to exchange it.anymore advice would be appreciated thanks