Hi we are having the same issue, we have set the custom WYSIWIG CSS to 980px, and can see in the editor screen that the change has occurred, only problem is, the content that is entered still only spans approx 600 wide before it goes to a new line, effectively it looks like this:

|content content content —blank space |
|content content —blank space |

When is should look like this

|content content content content content|

This is happening in both firefox and safari for me.