You need a new cassette. Looking at gear-inch numbers won't tell you anything more than a simple comparsion of 23/25 = .92. That means that you get 8% lower gearing with a 25T large cog.

As for the chain length, that's been asked and answered nearly every day on this forum and the answer is always the same - 2T more gear requires only 1/2 inch more chain, but the minimum increment of change is 1 inch. Whether the chain needs to be any longer depends on how the length was determined and your chainstay length. If someone set the length by the big/big plus 1 inch method and there is no excess length, then you'll need another inch. If the little/little method was used and done properly, the chain will be long enough. You can usually tell just by how extended the RD cage is, when it's shifted to the big/big combo (with the old cassette). If the cage has a fair amount of forward travel left, it will probably work. Mount the new cassette and slowly shift into the largest cog. If the chain is eally too short, it shouldn't even shift into the largest cog, or the chain might start to jam-up. Just don't do this test out on the road, under a load and you'll be fine.

http://www.parktool.com/blog/repair-...-length-sizing