Wider page with Iframe

Hello.
I would like to ask you about something- I need to make my single post wider. I have Iframe in each news page and I want to make Iframe page wider. This “foreign” page must be on whole screen, and there must be only one slider on the page- Screenshot maybe will be helpful:

shot

This is my website single post:

single

I’m sending CSS code of single post:

/* 11. Postbox*/
.postbox { width: 600px; padding: 0px 0px 5px 0px; border-top: 4px solid #B8240D; }
.postbox h2 { margin: 2px 0px 5px 0px; font: 15px/19px Georgia, "Times New Roman", Times, serif; font-weight: bold; }
.postbox h2 a, .postbox h2 a:visited { color: #16387c; }
.postbox h2.normal { margin-top: 0px; font-weight: normal; }
.postbox ul { margin: 2px 0px 0px 0px; }
.postbox li { background: url(images/dot-gray.gif) no-repeat left 7px; padding: 2px 0px 3px 10px; font-size: 1em; line-height: 15px; }
.postbox li a, .postbox li a:visited { color: #16387c; }
.boxleft { float: left; width: 295px; margin: 0px 0px 0px 0px; }
.boxright { float: right; width: 295px; margin: 0px 0px 0px 0px; }
.date { font-size: 10px; color: #B8240D; text-transform: uppercase; }
.gray { font-size: 9px; color: #666; }
.readmore a { font-size: 11px; font-style: italic; color: #666; }

If you want any others scripts/codes, I will send. Thank you very much for help.