Perhaps you need to make it wider, instead of narrower? Try changing the setting from 600px to 700px and see what happens.
DING DING DING!!!!
Not only did this work, it made me feel incredibly dense for not thinking to even try this on my own. Either way, it works now and I really appreciate the help.
For anyone looking for a different, maybe more elegant repair, it appears to be an issue with Tiny MCE forcing the width of the box to the length of the editor bar.