For now I'm using my 8x10" cloth (Delta 50x60") for 11x14", but I'm planning to make a new one this summer unless I find one that's suitable--at least 72" on the long side to go all the way around. B&H has one that's 58x72", but it's nylon, which might be too lightweight to be practical outdoors. There is a Wista that describes itself as "Large" and "for larger cameras," that looks more substantial, but it doesn't say how large is "large."

By the way, if you look at the B&H website, you have to search under "focusing cloth"