What size image should I upload for a "hero" image on a single post page?
The "hero" or featured image on a single post page should be at least 1170px wide.
Landscape format images are also recommended, with a standard size/aspect ratio (4:3 or 3:2 or 16:19) for consistency.
For more on adding featured media to posts, see the relevant section in the
Largo User Guide.