I think 16”x16” is the right size so you can fold it in to 4ths but then the towel needs to have the same towel material on both sides. For instance, I don’t like the Griot’s Crinkle towel as it is 16”x16” but has different material on each side so it isn’t really usable completely when folded. It is a little too big when unfolded and not fully usable as intended when folded. So if I use the Crinkle towel, it is part of a 2 towel system, Crinkle to wash and another to do the final wipe.