Article Page Width (Make Bigger)

I would like to make the width of the main article portion of my post pages 750px (for my website called PantrySpa.com). I would like the main body width to be 750px so that I can put an ad that is 738 px at the top of a post page. How can I change this width?

I have granted support access via the wpmu support plugin, in case that helps.

Thanks!