I like something which is at least as wide as the poker sized decks I am using.
My game has a dealer, with the button giving the previous deck their best shuffle before it goes back to the dealer for a final riffle-cut-riffle or whatever.
The deck to be shuffled sits on top of the button, so it is good for it to not be constantly sliding off. Also important for the button to be thick enough that it is clear it is under the deck.