For those who want a fixed size, adding a body entry to the Stylish script, like so:
margin-left: 0px !important;
might do the trick. Adjust as you wish. Posts that are too wide extend beyond the specified borders.
I haven't tried it everywhere in the forum, but it seems workable at a shallow glance.
I don't use Greasemonkey, so no guarantees, but I imagine something similar to the following line might work:
document.body.width = '800px';
-- Daniel Terhorst