First of all, if that's a full block ... those are QST not HSTs
Each 1/4 section of the 12-1/2" block (unfinished size) would be 6-1/2"x6-1/2". So each QST would be this measurement before sewing into the quilt block.
If the 12-1/2" you have given is for the finished block (after it's sewn into the quilt) you would need each QST to be 6.75"square.
Same measurements if you are using HSTs instead of the QSTs