Sheets of paper used on printing presses come in several standard sheet sizes. The job is printed, trimmed and folded and there you go. If your finished printed piece is of standard size then the press fed sheet area is maximized. If you go off the grid and have a non-standard size, say 12 x 8.5 inches, then more paper is trimmed off as waste, more paper is used, perhaps only 3 prints per sheet and not 5 prints per sheet. I’m all about standing out in a crowd, but do it with design and content not odd sizes. And as my mantra if you have your heart set on something unique, talk to your printer.
And for the geek in you, samples of standard sheet sizes are 23 x 35 inches, 25 x 38 inches and 35 x 45 inches.