Not really a wordpress question, and except for the last option you might get a different and maybe better answers on stack exchange.
Three options one ugly, one requires extra work and one is super hard but might be the right thing to do.
Ugly: Put the external object in an iframe and write some JS that will autommatically adjust the width and height of the iframe to the avaialable space. bonus points for handling browser resize events
Extra work: Use some Jquery code (Don’t have to, but that is what everybody does) to load the external object into memory. Parse it and insert the relevant part into a div on a page (use a shortcode to indicate the location of the div for non techi users)
super hard: change the external site to support oembed and return the relevant html for requests, change your wordpress to recognize the external site as a valid source for oEmbed. In the post insert the relevant url on the external site.