Hello. Here's my answer. The design you gave us is what I have always called an "hourglass". In order to keep the bias from stretching so much, it is suggested that you cut your squares 4 ways--quarter square triangles (QST). To make QSTs, your squares must be 1 1/4" larger than your finished squares. So, if your 12 1/2" block is one hourglass, then you must cut your squares 13 1/4" and cut them 4 ways. If, as your pictures suggests, you are using 4 hourglass blocks to make each 12 1/2" block, then you cut your squares 7 1/4", cut them 4 ways, and after sewing a bunch of hourglass blocks (which would make 6 1/2" blocks unfinished), sew 4 together to make your intended block. When I want really scrappy ones, I use graph paper to make my large block (like 13 1/4"), mark it for the QST, cut them apart, then I simply place one on my scraps and cut. I do try to be careful to put the long edge on the grain so that it is not so stretchy. The blocks fit together so much better using QSTs. Ask me how I know??? LOL!
Joyce