Hey TarStaff,
Hope you’re doing well today!
If you open up your functions.php file and go to about line 214 then you should fine this:
add_image_size( 'single-thumbnail', 470, 260, true );
screenshot illustration: https://screencast.com/t/d5l55esg
Those are the width and height of your featured images. Change those to whatever you like. In this case you will likely want 750px width as that is the width of your post content.
You will want to make sure you upload pictures that are at least 750px in width too of course.
Keep in mind this will only change your featured images from now on. Your past uploaded ones won’t be changed because they have already been cropped. You can always go back and re-upload them though ??
Hope this helps TarStaff!
Cheers,
Tyler
PS. Not sure if you have a child theme here but this change would be best made in a child theme, otherwise you will need to re-do it after every update. https://codex.www.ads-software.com/Child_Themes