I made an example here
http://jsfiddle.net/sheldon_griffin/a3ncx/1/
In short, one way to do it would be a css rule of
.ui-btn { padding: 10px; }
I'd strongly suggest adding an id to your button container, though, and restricting the bigger buttons to that container - in some cases, other buttons will look strange because of the increased size.
So if you added id="buttoncontainer" to line 3, you could use
#buttoncontainer .ui-btn { padding: 10px; }