Hi,
What version of mojoPortal are you using? You can find out from Administration > System Information.
If you are not using the latest version, I would recommend upgrading.
Really the editors are all configured to use 100% width so the width is determined by the width of the container div. That is why we only have a setting for editor height. You might try using a different skin and see if the skin is a factor. I would also navigate away from your site, clear the browser cache of temporary internet files then go back and see if the editor works. I would keep an eye out for script errors which may be indicated at the bottom left of the browser.
You might also try the demo site, I don't think you will encounter any problem with the editor there.
Hope that helps,
Joe