In the fancybox homepage (http://fancybox.net/home) there is an example that opens an iFrame dimensioned as the 75% of the screen.
I can't get it by modifying the width and height properties on the .js file as described on the site.
You should try this:
$('iframeLink').fancybox({
'width':300,
'height':200,
'type':'iframe',
'autoScale':'false'
});