Still not working (code was transformed to an actual link) but I think I understand what you are asking…
Passing parameters via the URL like ?width=800
will not do anything for FancyBox. If you want to change the light box size, you need to (1) enable the option “Include the Metadata jQuery extension…” in the Links section on Settings > Media and (2) add parameters to the link class like this:
class="fancybox-iframe {width:800,height:600}"