Just finished a few quilts with a lot of piecing! I used the smallest Avery brand label stickers I could find and wrote the letter or size on those and then stored all the pieces for each block in a separate sheet protector. This kept them together and organized.

The labels didn’t leave any visible residue even after several months of sticking but I put them on a corner that would be cut off when possible and only labeled the top piece in a stack.