OK, so 800px is a good content width as after padding the width would be around 720 – 760px which creates a nice reading width for paragraphs of text.
For image sizes, you can upload a larger image. If you’re not thinking about full screen width images then 1200px is good. For full width you may need to head towards 1920 – 2400px wide.
When uploading images to WordPress, it automatically creates what are called srcset image sizes : Thumbnail (default 150px), Medium (default: 300px) and Large (default 1024px). WP will use the size that is most appropriate to its needs.
Also GP has its own resizer for featured images, you can adjust the featured image sizes for Posts / Pages and Archives separately in the Customiser > Layout > Blog. Or leave them as the default Auto but always better to have a larger image that can scale down.
So if you’re thinking you may want to display the images larger, say in a GP Page Header then go for 1200px ๐