Adjusting menu width on my site

A browser tool like Firebug should show you the CSS for any element on the page – in this case see:

#main-nav {
    background: url("images/navbg4.png") repeat-x scroll 0 0 #E9E9E8;
    display: block;
    float: left;
    margin: 0;
    padding: 0;
    width: 1000px;
}

If you need additional help, you should check with the developers of your theme – as these forums only support themes from here:

http://wordpress.org/themes/