I think you kind of have to play with it and see what you like. The advantage of using larger size tights is that you get more headbands out of it for the money. But, there's also a lot more fabric which make the bands bulkier. The smaller sizes you don't get as many head bands out of, but they are less bulky for smaller baby heads! I usually end up using about a size 18 months, and getting 2 bands out of it---one from each full length leg. Not really a definitive answer, but I hope it helps a little!