how to customise the width of #cboxLoadedContent in jquery colorbox

I'm using jquery plugin colorbox,

I encounter a problem when trying to set width and height of div #cboxLoadedContent which is a child of cboxContent div, by default it using box-sizing, I don't know how colorbox calculate the width & height of this div. I have for example the parent div:

<div id="cboxContent" style="float: left; width:572px; height:396px;">

<div " id="cboxLoadedContent" style="display: block; width: 570px; overflow: auto; height: 394px;>

Can I have the same value of my cboxcontent in cboxLoadedContent?

Thnaks for your answer.

In fact I'm not using rel:'group',

This is my code :

var popin_width = '572';
var popin_height = '416';
if( $('.ie8, .ie7').length )
{
    popin_height = '417';
}
if( $('.ie7').length )
{
    popin_height = '427';
}


$('.popin').colorbox({
    inline: true,
    width: popin_width,
    height: popin_height,
    opacity: 0.8,
    overlayClose: true,
    close: 'Fermer',
    onOpen: function() {
        $('a, :input, object').attr('tabindex', '-1');
    },
            onClosed: function() {
        $('a, :input, object').attr('tabindex', '0');
    }
});