Increasing Spry Button Sizes

I have a feedback form that works fine. I need to create the same form that will work on a smartphone. I have created it and it looks fine apart from the fact that the spry buttons show up so small that it's difficult to tell whether one has been selected without zooming in on the phone. Could anyone tell me how to make the buttons much larger - without increasing the size of the associated text - which looks fine in it's current state. The development version of the page is here www.hollisterairshow.com/mobile/feedback2.php

Thanks

Tony