I have been teaching a method of making 4 half square triangles at once for a long time. My formula is to measure the long side of triangle pattern piece. For example if the long side of your pattern piece measures 5 and 3/8 you would cut your two squares 5 3/8". Once you sew all the way around (1/4" seams) and cut on both diagonals you will wind up with half square triangles which will measure 3 3/4. Hope this is clear, if not let me know and I will try another approach.
Renda