I don't know if you can resize things, or if you can you need to do more than I know how to. I think you're unable to change the file size of it, though, so if it's something like 48.0KB, then the file needs to stay 48.0KB, otherwise the game won't be able to read it correctly, and it comes up looking like that. I've encountered this with decals and the Octagon Arena reskin I was doing recently, as well as the UI I've been messing around with, so that'll probably be what's wrong with it.