In "classic" (non-Artisteer) mojoPortal skinning, the following CSS will handle all of the widths of the page, colums, and center (example taken from the mitchinson-golden skin):

#wrapwebsite { width:970px; }
#wrapcenter { width:969px; }
.leftside { width:224px; }
.center-nomargins { width:968px; }
.center-rightandleftmargins { width:494px; }
.center-rightmargin,.center-leftmargin { width:731px; }
.rightside { width:224px; }

You need to be aware of the interrelationship of all of these selectors. They nest like this:

wrapwebsite (970)
   wrapcenter (969)
      .center-nomargins (968)
      .leftside (224) + .center-leftmargin (731) = 955
      .center-rightmargin (731) + .rightside (224) = 955
      .leftside (224) + .center-rightandleftmargins (494) + .rightside (224) = 942

So, if you want to expand the left column, for instance, you'll also need to adjust the other settings downward accordingly. The numbers don't all add up to the same width, because space needs to be saved for margins on each of the columns and the center.

If you install the Firebug extension for Firefox, you can view the CSS on the page and play around with adjustments to see how they affect the page layout. It's really handy for prototyping.

Jamie