You'll need to call colorbox manually on the item for this to work.

Set up your element with a custom class, like "cbCustom", or something of that sort. Then all you need to do is add a custom call into a script file in your skin, or in a script tag in the layout master.

$(".cbCustom").colorbox({
    width: "500px",
    height: "500px"
});

This will make the colorbox element open with 500px width and height. You can change that to read whatever you want, and it will accept percentages as well.

You can find more information about colorbox here: http://www.jacklmoore.com/colorbox

Hope this helps,
-Isaac