In reference to option 2, in the content.php file is this line:
You would make it this:
That would make the “Featured Image” show on the full post view.
If you make that change, I recommend you do this with a child theme so that you can get updates without losing what you changed. This theme does include a child theme that you can install and use. Basically you would copy the content.php file into the child theme then make that edit there.
For images and sizes, you will want to see this tutorial: