My standard requirements when looking for a wrist rest:
-Flat, not rounded like the gel type
-As tall as the front lip of the keyboard case (about 3/4" or so for most 'boards)
-Generally rectangular in shape (this is more aesthetic than anything)
-As close to exactly the length of the keyboard; a little shorter is fine but not longer
-The wider the better. My Milestone is about 3" wide, and I find that to be a great length
Obviously, this is what I like, YMMV.