I can tell you what you're doing wrong in terms of Seth, but if you just want to keep Jimmy's height:
Backup the ide first.
Copy from Medium until "/Anim/Player.act.." and then go down to Seth. Highlight "Huge" until "Authority.act.." (make sure you get all the dots before the slash), and paste insert. Now in another ide, copy from "Huge" until "Authority.act.." and paste insert onto Jimmy's thing (Medium until Player.act..).
And it should work.
If it still doesn't work, I'll upload one for you.