Probably by resizing all the different frames individually.:(
-Edit- OOH! Or you could use the resizey-do feature on Imageshack. If you're uploading the GIF, and tell it to resize it, it should shrink it down... Or blow it up. It might work.
The problem with animated PNG files is that they can become quite simply huge in terms of space taken. I tried making one to combine two signatures with liberal use of transparency, but unfortunately the size exceeded three whole megabytes. If it takes an hour for people to load the image, it isn't worth it for me to have it on.
(Of course, if you can manage a smaller file size, then that isn't a problem.)