You'll have to play with the CSS of the skin.

And you'll probably want to copy it before doing so. This is the first step in creating your own skin. So, be careful.. ;)

You realy are better selecting one of the "full width" skins
example : http://skins.b2evolution.net/a.php?skin=Classic

The Custom skin is designed as a centered fixed width skin. You could edit it but you will find yourself having to do a lot of other edits as well to make it work as you wish.

Plus the custom skin uses background images as part of it's visual deal. To tweak it via css would require you to tweak the images unless you're willing to accept an ugly blog. (So far no one who tweaked the css was happy with an ugly blog and had to learn to tweak the images as well...) There are a couple of skins that scale to fit the monitor your visitor is using. "Classic", that John linked to, used to be the custom skin way back in the day, but there are others. Poke around the skins repository and see if anything strikes your fancy.