I tried to make a larger boxy bag a few weeks ago. I cut out the dimensions like it said. The finished bag was 4 inches tall but was too wide and long.
The second attempt I cut off more of the bag and made it shorter. I thought cutting the corners smaller would make it taller. Nope! It came out 1 inch tall! I was about ready to throw in the towel, but decided to give it one more shot. I cut it down to 10 inches wide and kept it 15 inches long. I cut the corners 2 inches starting at the seam lines. It came out 4 inches tall. And looks exactly how I wanted it to be. Tall! I had used a 14 inch zipper.
When I went back to read the dimensions it said, 15 by 10. I could have thought it said 15 by 18, no wonder I messed up. So, that's why I chose 10 inches. But when I went back to the site, it said 18!??
Anyways, next bag will be 10.5 ( to allow for seam allowance) by 15 and sew the zipper on the 15 side.
I think I've figured out the secret. You cut the corners how tall you want it to be. So, since it was 10 inches wide and I cut the corners 2 inches off, then that leaves 8 inches. 4 inches on each side. 4 inches tall. I'm going to test out this theory later on when I make another bag. The finished bag was 9 inches long, almost 5 inches tall and almost 5 inches wide.