What does "11-25" mean in the cassette description?
The first is then number of teeth on the smallest and the second is the number of teeth on the largest cog.  

If you like the current gearing then stay with that.  Or you can adjust.  If you stay with the same then match the number of chain links.

The count is also typically also stamped on the cogs.  On the bike and dirty it is typically easier to just count them.