Rule of thumb that I have learned is with good top and bottom support to keep your window from bowing the length or front to back doesn't matter much.. I only believe this to an extent. If your going with a 16 foot solid window I would say you need to be alot thicker but if your going with 2 windows at 8 ft with a good solid support in the middle and top and bottom I would say 11/4 is enough. But im no expert