Enforces a maximum page width per post type in the WYSIWYG Visual editor so content will wrap according to the design of the front end.
In the admin section, navigate to the Settings => Writing section. There you will find the "WYSIWYG Editor Page Widths" section that should list all available post types by their name, and respective fields for setting the max width in pixels.
Because I said so? :D Really because why would you need any other setting? In all seriousness though if you need a larger (or smaller) dimension just let me know and I'll update the plugin.
Well, that adds to the complexity and most layout widths are not measured in EM's. 960 Grids are easily converted to pixels so I used the lowest common denominator.
I suggest you look into using a DOM Inspector like Firebug or IE Developer Tools. Both will give you an option to hover over the content on the front end of your website and select a paragraph of text. Then you can check the computed width of that content (here's a link I found on the web that should get you started: http://www.evotech.net/blog/2007/06/introduction-to-firebug/) Alternatively you could just try a series of sizes or use a piece of paper against your monitor to eyeball the measurement between the front end and the backend.