I recently got a Nashbar carbon road bike with a Shimano 105 rear derailleur and a 12-25T cassette. I am quickly learning there is no way I'll be able to do any kind of steep mountain rides with the current gearing I have.

My question is, what is the largest cassette a 105 derailleur will accept? I'd like to go at least 28 or 30 or bigger if I can. Thanks

My bike:

Nashbar Carbon Road Bike - Road Bikes