I've never understood why the size variables get greyed you when embed and in frame is selected in the URL appearance section. Looking at what Moodle does I found that it's simply an iframe that gets inserted. So what you could do is to create a page resource instead of a url resource and insert an iframe with the desired dimensions. The Moodle documentation actually contains an article about iframes.
https://docs.moodle.org/28/en/Iframe
You may have to experiment with the iframe tag id to make sure it gets treated the same as an embedded url regarding responsiveness to window size changes. The good thing is that you have control over things like the frame around the embedded page.
Hope it works.
Christoph