Niniel

It depends how he gave it to you. If it'S the raw file, with layers, you might be able to resize everything.

If it's just the .gif file you either have to redo the whole animation in the new size or tryo to go with image/image size and enter the new dimension in the menu... Keep the "constrain proportion" checkbox checked to avoid having it stretched (but you might not be able to have the exact size you need by doing so)