What size finished component do you want?
It might be easier to start with that -
Something else that you might do - mark off the sewing line on the long edge of the triangle- is that the side that's to go on the inside square?
Measure the stitching line - I'm guessing it will be somewhere around 14.5 inches -
Cut a square that is the stitching line plus 1/2 inch for the seam allowances.
When you sew the triangles on, there will be "ears" sticking out -