I don't know the exact way to do it in Pub, but keep in mind - not
everyone who views your web pages will have the same resolution as you.
Recent surveys show slightly above 50 percent of web viewers have a
resolution of 1020x768 or higher. That means that just under 50 percent
at at 800x600 or lower. If you make your pages over about 790 pixels
wide, lots of viewers will get a horizontal scroll bar, which will not
make your site as viewer friendly as it would without the scroll bar.
Mike