I went a couple of inches bigger, but the girth belt won't actually fit all the way around your waist, anyway. Specifically when you've got layers of armor on. You're going to have to use some 1" nylon strapping to buckle it behind you:
Boba Fett Empire Strikes Back Costume - The Dented Helmet Gallery
Just make sure you get real mohair. I got mine online at Dover Sadlery, and it's perfect:
http://www.doversaddlery.com/product.asp?pn=X1-02174&ids=475993510
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