I've tried playing with a few settings: adjusting the width of the editor, using screen CSS, auto-sizing the editor, using custom CSS to set the editor's body to 100%... no luck.
Here's a screenshot (I've reset all settings to default):

I'm using CMSMS 1.6 and TinyMCE 2.5.0. I'm a Mac user and have tested this in Safari 4.0 and Firefox 3.5.
Thanks for any help.