According to the (conflicting) documentation of TinyMCE, the editor takes on the size of the textarea (or other) element that it replaces. It also says that you can set the
I noticed that a containing table was enforcing widths, and also the icon set are td's, so there's a minimum width they'll collapse down to, so if you have many icons in a single row - it could be messing all over your widths.
Sort of related SO question of mine, ended up being quite related to your problem: Able to float td elements consistently?