To end up with a block that is 40" finished, the individual blocks would have to be 5 inches finished, 5-1/2 inches unfinished.
Using Marilyn's numbers, instead of 2" squares, you would need 5.5" squares. Instead of 2-3/8" squares, use 6" squares. To make the half square triangle blocks, I would start with 6" squares and trim them down after sewing the diagonal stitching. Then to make it 50x50, you would need to add 5" of borders. I would use one small border (say 1.5")and one larger border (say 3.5")