11 x 15 inches is a convenient size for painting watercolors, especially for beginners. I buy a 22 x 30 sheet of my favorite paper, Arches 140 lb cold pressed paper, and tear it in quarters, to get 11 x 15. This file shows how do that. Most important, be patient; keep folding the paper back and forth until it tears without great difficulty. You can view this online or download it for free.