The instructions are for 2 different sized finished projects. The pieces may be used in different final settings to achieve the finished sized required. Just choose your size and go for it. Another way to do a lot of HST is to take a strip of both fabrics, half the width of the fabric and the width of the block you need for the finished HST. Press them good sides together. On the back of one, mark the blocks, mark the diagonal line in a zigzag pattern, then sew up one side and down the other. Set seams, cut blocks apart and then cut the HST. (If you plan to square down, use a little bigger square, I use Eleanor Burns SquareUp Ruler, before I press open.) Press open. And you will have done six at a time. This is like making your own Triangle papers for any size HST you are making.
Charlie