How to check length of body element in iFrame

I have an iFrame that I would like to check the length of the body element within in it. My iFrame looks like this:

How to zoom and preview only specific element in iframe?

I’m using <iframe/> to show live preview of my cards.

Reloading a React page containing an iframe loads the iframe itself, not the React page

I have a React page that uses an iframe to show a map with various points marked on it.