should be an 12-25 or 12-26 (whatever everyone else had, mine was slightly different) but if you rub the cassette clean (find the first one and follow in a line) you will find each one is stamped with what it is followed by a T...
Quite a few of us have fitted 11-28 cassettes now which have been great (and those extra 2 teeth really do make a difference), some needing adjustment, some without, but the existing derailuer is only meant to take upto a 26T and I am certain will not go passed a 28T given I have the 11-28T (SRAM PG850) and have had to adjust the clearance of the derailuer to max (B screw). Good habits also dictate a new chain when the cassette is replaced, no matter what, though some don't, I would recommend it as it will rule out any issues with chain slipping later on leading to an easier diagnosis of indexing issues after a few weeks which I experienced.
also went 11-28.... found I needed to change the rear deraileur as imwasnt happy it fitted well.