Your main content widths are being controlled in these two elements:
#contentwrapper {
float: left;
padding-left: 5px;
padding-right: 20px;
width: 505px;
}
#sidebar {
float: right;
margin-left: 15px;
margin-top: 0px;
padding-top: 5px;
width: 350px;
}
The simplest way to make your sidebar wider is to increase its width and decrease the width in “#contentcontainer” by the same amount. This will minimize the amount of additional width changes you may need to make. For example, change the sidebar width to 360px and reduce the contentcontainer width to 495px … this may adversely affect other elements so test thoroughly after making the changes.
Hope that get you pointed in the right direction …