Like bcr said, just get the larges. I got Dottie large ones when she was about 6 months old and just left them as is and she was fine with them being a little big. She grew into them. Now though, they are way too small for her as most of the time a coil or 2 hangs out from under the hides lol. I just ordered jumbo ones from RBI which should give her plenty of room.
As a side note for size comparison, I got 2 mediums for my 30" dumerils and she fits in those. But I'll end up giving her a couple larges as I'll have 4 extras from Caesar and Dottie.