First goto CSS folder into the theme and type CTRL +F, it'll show a new prompt or will appear at bottom of window. Type div#wrapper there and it'll direct you to there. (If prompt, click find to get directed) Once you get there you'll see this code.
/* This division wraps the entire forum when a forum width is set. */
div#wrapper
{
margin: 0 auto;
width: 950px;
}
And simply edit the width you want.