download GIMP, its a free open source Photoshop like program.
open up the GIF you want to resize into GIMP
then on menu click "image" then "scale image"
then choose what width and height you want to change it too and save as a GIF.
if your talking about adding the function so the thumbnail created is a smaller animated gif instead of a jpg or the resizing of a animated gif file keeps it animated when you resize in chevereto the im unsure but im guessing its alot of code.
but if thats what you want then im sure someone could make you a mod for chevereto if you paid them.