Yes there are many automated GIF resizers available on the web, but the advantage of using Photoshop for this task includes having the best output quality with the least file size. Not to mention you may also do some enhancement such as changing the brightness or sharpness of each frame.
1) Go to File > Import > 'Video Frames to Layers...'
2) Browse to the folder which contains your GIF file. If you cannot see the file do this: Under 'File name' type *.* and press enter. This will refresh the folder and reveal your file.. select your file to load it.
3) 'Import Video To Layers' window will open. Press OK.
4) The animation will be loaded and you may go to Image > 'Image Size...' or Image > 'Canvas Size' to resize or crop the picture to your desired size. You may further edit the photo (such as changing brightness or color) using other photoshop tools but that is not covered in this guide.
5) When you're done, go to File > 'Save for Web & Devices...'
6) Make sure GIF format is selected in the right hand side drop down menu. Finally click 'Save' to save your new GIF file!