Width of Text Area Box in Editor

I REALLY love this little bulletin board extension! It's just what I need to be able to embed into my art site.

BUT, and I know this has been asked before but I can't see where anyone has answered, so sorry if I'm being redundant, I JUST want to change the WIDTH of the text area in the New Topic post...I've tried to find this information myself, but I cannot find how to change it from 500 pixels wide to something more suitable for my site (like 425 pixels).

If ANYone has been able to fix this or has code that I can use to change it, I would be so VERY grateful! I've had no issues whatsoever customizing the template for Joobb-black, but this one thing is just driving me crazy!

J. D.