Best Featured Image Size?

I’ve been going through the forum and tried all the suggestions I’ve found but no size I try works for (1) main top image of listing, (2) Related Listings, and (3) Recently Added.

Not matter what size I try, one of more of the above will show the dark grey background color.

So, what is the right size? Or alternatively, what CSS should I apply so one size works for all?

Thank you.