It is only possible to do this cross domain if you have access to implement JS on both domains. If you have that, then here is a little library that solves all the problems with sizing iFrames to their contained content.
https://github.com/davidjbradshaw/iframe-resizer
It deals with the cross domain issue by using the post-message API, and also detects changes to the content of the iFrame in a few different ways.
Works in all modern browsers and IE8 upwards.