Start by estimating what size you normally wear in a pant or sweatshirt....M, L, XL, etc. This will get you close.
Then look at Inseam. This is measured from crotch to bottom of pant leg. Is it approximately what your pant inseam is? It is OK for the wader inseam to be an inch or two longer than your normal inseam.
Does your street shoe size fall in the range of sizes shown?
Finally, look at the chest/waist measurement. It is OK if these are a few inches bigger than what you actually measure. A bit of extra room will allow you to layer under the wader and waders come with a belt that will take up any extra fabric.
If all this is not working, just give us a call at 907-586-1550. We have fit hundreds of anglers and can help you sort out any wader issue.