Are you saying you want to increase the site width on a specific page, or that you want to get rid od dead space (like paddings and margins in the content area of a specific page?
if you want to increase the site width on a specific page, you can add the rule below in Main Options > Fonts & Custom > Custom CSS Rule box.
.page-id-xxx #wrapper {max-width:1200px;}
replacing xxx by the page id #
If not, please elaborate and provide a link to the site so we can be sure.