I've just measured my little boy as I'm wanting to buy a UD nappy.
These are the measurements I've come up with:
Waist (just under belly button) 17"
Rise (from where his usual nappies sit) 13" but nappies with a 15/16" rise fit fine
He's 5.5 months now and he is chunky but not abnormal. Looking at the sizing chart on UD his waist means he needs a large but that can't be right surely?