Why do you want bigger rings? Unless you're spinning out in your current high gear there's really not much point in the swap, and commuting speeds are rarely high enough for that to happen.

Sheldon Brown has a nice gear calculator that'll give you top speeds at various ratios and cadences, check it out.

If you haven't got a commute that's out of the ordinary you'd probably benefit more from going to a road cassette than messing about with your rings.