Hi Michael,
For that type of block, images with a 1:1 ratio (square format) would be ideal e.g. 400x400 px or 600x600px. You can also use a landscape image (400x250 px)
For example, for the 'Subscribe to newsletter' banner in the Totara Community, we use a 400x400 px banner size with a Landscape Shape and tile size set to Medium. For the 'Learning with Impact Takeaway' banner, we use a 600x600 px with a square shape and tile size set to Medium.
If you want to use the Portrait shape on the side panel, I recommend going for a 400x640 px banner size.
Many thanks,
Beatriz