Because I'm impatient, I ordered the small -- because it was the only one in stock at Performance Bike -- knowing it would be, well, too small. I figured I'd be able to rig up some sort of workable extension.
Luckily, I figured right. I purchased some industrial-strength velcro to add to (what I believe is) the front. With the extra few inches, the belt fits comfortably around my hips, AND puts the bottles low enough and farther back enough that I won't constantly hit them with my arms.
The belt has a small pouch in front and a larger pouch in back to carry keys and -- I hope -- a Hammer gel flask. If I use it to bike, the pouch is also perfect for my mp3 player.