If you buy a sheet at 22 x 30 in, you can cut it to 15 x 22in and have plenty of room for borders (which is quite useful for hand coated paper). You could also cut it down further, but I'd leave about 1 1/2 - 2 inches around each edge, but that depends on how you are coating the paper.

11 x 15 would be really difficult to print to the edge, but you might be able to manage it with careful coating. Be prepared to get chemistry on your coating surface, though.