I believe Walmart is sourcing theirs from Optics Planet and they are a fine company to work with. At the web site below they have a sizing chart which will assure a proper fit. And they are also the lowest price retailer I am aware of. I will be surprised if yours takes a larger one than Medium or Large at most. All of my English game guns use the Medium.
What I really like about a removable pad is in the summer I can slip it on when shooting clays or whatever but during hunting season with extra layers of clothing I leave it off. Instant stock adjustment!
http://www.opticsplanet.com/galco-recoil-pads.html?_iv_code=GC-RC-EFFAAF-LT1032DH
