Hello, Abbie!
The easiest way to solve this, is to turn off the Featured images on a single post (Customizing->Blog->Single Post)
https://prnt.sc/o0aejb
And Insert again your Featured Image in the Post content.
https://prnt.sc/nrddgk
In that way, you can change the image size per your needs.
For the youtube videos, use the next code:(insert it into Customizing->General->Additional CSS)
.wp-block-embed-youtube{
width: 700px!important;
height: auto!important;
margin-left: auto;
margin-right: auto;
}
Change the width value per your wish.
Regards