That’s what he’s referring to @bugbuster! The inputs currently use Ace, an embeddable JS code editor, so we have to work within the options that Ace can give us.
Personally I think those editors would work well. Because for me, it was always the height that was the problem, not the width of the editor. This auto-expanding solution takes care of the height problem.
Makes me wonder if, to complement this change, the Post by Email box should then go above the Custom CSS/JS inputs, right below Text Rendering. But then again I don’t think this is too drastic of a change to affect anything else on this page (unless I am missing something).