You basically got it. You can change the size of the pix as long as you make a new animation using that pic size. It may give you some file size gain, but most forums will not accept an Avatar unless both conditions are met...file size and pixal size. I've got all my animated pix listed by name with pixal and file size, yet when I picked one that meets the pixal size and it still exceeds the file size, it's rejected. Vice-versa too.
Perhaps I have to change the size of the animation and not just the size of the pic that represents the animation?