Just measure your snake (approximately + a little "wiggle room") when he's curled up. You probably don't need 2 different sizes, but you do need 2 of the same size- & IF they're a little big too big, as I said, you can wad up a paper towel or 2, & stuff them in one corner to make them seem more snug as he grows into them in time. If you just ordered them, you have time to cancel one size or the other, just get 2 of the size slightly bigger than he is. It should help a lot. And face them sideways (toward the center) also, not facing forward where you're looking right in at him- that would be like having people looking in your window...not sure I'd feel like eating then either, lol.