I don't deal with animated gifs, so I'm going to assume that you have a way of getting the separate images out of them first. So what I'd first suggest is to convert the image to gray level by taking the red channel. Then call bwconvhull() with the 'union' option to get a sort of round blob. Then I'd find the diameter by calling bwboundaries and finding the greatest distance by passing the boundary (x,y) coordinates into pdist2() if you have the stats toolbox. If you can't figure it out, post just one of the frames and your code and I'll fix it for you.
You might also try imfindcircles() and see how that diameter compares to the pdist2() method.
