first pair of plastic pant/diaper cover (help/suggestions?)

So i'm considering buying my first plasticpants/diaper cover to go over some disposable diapers, but i'm currently feeling very lost. I'm looking on northshore and Garywear seems to be the best rated ones on their site. I'm a little bit confused on the sizing chart, and dont want to get the wrong size and loose $25 due to the no refund ordeal after being opened. So i just want some clarification that i'm correct. If someone has a suggestion for a different pair of plastic pants i'm open to suggestions, my biggest concern is the sizing and that they dont cost anymore then $25

http://ift.tt/1HWkIK4

Waist: 36 1/2
hip: 34 1/2
thigh: 21 1/2

I think i'm an extra small, according to their sizing chart but not 100% sure. Should i measure with a diaper on? or without? measurements above are without a diaper.

first pair of plastic pant/diaper cover (help/suggestions?)