Autodetecting a size of IFrame object on the page is done via javascript calls. Unfortunately, such scripts are only working on same-domain pages. Cross-domain javascripting is disallowed by browsers for security reasons.

There are some (fairly unreliable) techniques out there - some employ Flash, for example. 

Of course, if you can plant code on the IFrame'd pages directly, you could write some code to communicate back and forth - but, I am guessing, this is not an option in your case.

One way you could definitely achieve the desired result is to use a overlay / window pop-up instead of an IFrame...