My cousin used medium-size moving boxes when she made this, but you can use any sized boxes that you want. If you do use a different sized box than what she used, you'll just have to adjust your measurements accordingly. It doesn't matter what size box you start with as long as you make sure to cut all four sides to the same size. If you don't, the sides won't match up when you're trying to tape them together and you'll end up with a gap. Be sure to keep the size of your box in mind when you're ordering your letter stencils. If your box is too small for your letters, you won't be able to read them from afar. And honestly, if your box is too small, you won't be able to notice it from afar either.