I just edited this page (using the Web Developer extension in Firefox) to display at 750px. I'm typing in the fast reply box and everything on this page looks swell.
CODE
#ipbwrapper{
margin: 20px auto 20px auto; /* centers the box, no matter the overall width, also applies a 20px gap at the top and bottom of the board */
text-align: left; /* re_aligns text to left second part of two part MSIE centering workaround */
/* EDIT THIS TO CHANGE THE WIDTH OF THE BOARD -> 750px is a common fixed resolution size */
width: 750px;
}
Okay, I've just switched to the Standard Editor and edited the page to 750px again, and the width of the page around the editor section looks to be 830px (according to the "MeasureIt" extension anyway). Is this what you mean?
You can go into your ACP and then:
TOOLS & SETTINGS > Topics, Posts and Polls > Post Editor's Width: {change to 100%}
You might want to do the same for the PM Editor's width as well.
HTH,
Rick