I was able to resize it fine to 20x20 in FW. Do you have the
animated GIF? Or are you trying to copy it from that URL?
Here's what I did:
1] Went to your URL, right-clicked the GIF, and selected Save
Image As -
saved it to my Desktop.
2] Start FW, choose File > Open, and open the GIF
3] Switch to the Frames palette/tab and verify both frames of
animation are there
4] Choose Modify > Canvas > Image Size (or Ctrl+J)
5] Unchecked Constrain Proportions, then set both width and
height to 20px
6] Click OK, then choose File > Save
Adobe Community Expert