For silver gelatin, I would probably go 16x20, but I would trim off an inch on each side (down to 15x19) so that I could easily use 16x20 trays for processing. The emulsion of silver gelatin paper tends to chip at the edges if not handled carefully, so it is nice having a little extra around the image.

For platinum printing, etc, I would go with the 16x20 if found in that size -- or cut the 22x30 in half as previously suggested. Looking at the prices of COT 320, 16x20 is a little cheaper than a half sheet of 22x30