Photoshop doesn't handle gif files very well, though it's awesome for gif making. If you open it with PS you will only get the first frame and not the rest. You need a specific program or a website for that I think. (Google will get you some results.)
Gif Resizer is a free portable program for that for instance.
ETA: It's for Windows, so if you have a Mac a website, like GIFWorks is a good option.