i had the same problem - narrow content area in one theme. you can add a width setting to the tinymce.module in the root folder:
within the tinymce_config($profile) function (it's on line 517 in my rather vanilla installation of tinymce), i added the following line:
$init['width'] = '465';
depending on the plugins/buttons you're using, you may need to rearrange the order. you can do so by editing the plugin_reg.php file, also in the root.
