Install ImageMagick (
ImageMagick: Convert, Edit, Or Compose Bitmap Images) and use it from the command line or search for the ImageMagick bindings/wrappers for your favorite language.
For example, for Node.js, there's
gm : GraphicsMagick for node.js. Could write a simple loop that resizes your gifs locally.
Off the top of my head:
Code:
var gm = require('gm');
// Resize gif to a width of 100px (preserving aspect ratio)
// if it's wider than 100px
gm('example.gif').resize(100, null, '<');