Hi Jamie,
I was able to see this problem on my local machine using the artisteer-30verticalmenu1 skin.
For this skin I was able to solve it in style-artisteer-oveerides.css like this:
body.wysiwygeditor p
{
font-family: Arial, Helvetica, Sans-Serif;
font-size: 14px;
margin: 12px 0;
width:93%;
}
note that the only part I added was the width:93%; the rest of it was already in there.
Also note that the editor is resizeable by dragging the bottom right corner so it can be solved that way as well, and also if you have the cursor in there you can right arrow and as the cursor goes over to the right it brings the content into view.
Must be something funky with the artisteer 3 css to make it overflow like that, not easy to figure out exactly what css is causing it in the context of the editor.
Best,
Joe