Stocking foot waders, gore tex or neoprene, should fit comfortably over your wading pants. You don't want a "body glove". I recommend that you find a local fly shop and stop in and talk with them and try on a pair of each and allow them to size them for you. Make sure when you try on your wading boots it is with your socks and waders on like you were getting ready to go and fish.
Another thing on waders. Like most things, you get what you pay for. I recommend you check out a pair of Dan Baileys (try www.redshedflyshop
- site sponsor; carries them and has the best no nonsense approach to fly fishing of any shop I know). They are a good value for the money for sure. If you want to pay for Simms you could do that. However, I don't recommend you go cheap. You'll be very frustrated if you do.