If you made the set yourself and still have whichever art program you used then resizing it shouldn't be a problem.
If you bought it off someone then there's nothing you can really do about it.

Syv was talking about Upsizing, and that always ends in blurry stuff, no matter what program you use, downsizing isn't a problem and isn't necessary as toribash automatically downsizes 512 textures being uploaded on 128 slots, maybe it causes a bit of blur but it's easy to do manually if really needed.