The mechanic assistant forgot to mark the pistons before dissasembling so now we don't know which piston is which.
Where do you can find the 1 or 2 piston size sequence stamped on the block?
On what part of the block?
It might not be depending on motor and age. Since you don't tell us what engine, they need to do what they should have learned as apprentices, micrometers (internal and external) measure and calculate which one fits which bore.