You can use this library, which both initially sizes your iframe correctly and also keeps it at the right size by detecting whenever the size of the iframe's content changes (either via regular checking in a setInterval or via MutationObserver) and resizing it.
https://github.com/davidjbradshaw/iframe-resizer
Their is also a React version.
https://github.com/davidjbradshaw/iframe-resizer-react
This works with both cross and same domain iframes.
