Can you try adding div (see below) at the beginning and ending of your Youtube embed code:
50% would be the width you want, you can customize this to be other sizes (by percentage / pixel). I have tested on my development blog and it should work:
I updated the htmal to
<div style="width:50%!important"><iframe width="560" height="315" src="https://www.youtube.com/embed/FRBure6USpE" title="YouTube video player" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture"></iframe></div>
but I don't really see any changes on the website, so strange!
Do you know the reason why the video is on the top of the post (above the main image), instead of being between paragraphs?
Thank you so much,
Hmm you might have to contact your theme developer for support. Seems like there's some theme code overwriting what we have.