What image size is needed to display in PS?

Question

Search engines

I want to display an image in the search results.
1. Using og:image (Open Graph markup), what size should the image be?
I understand that you can set the width and height in the markup, for example 1920x1080, but is such a large picture needed?
Should the picture be square?
If the picture is 600x600 is it normal?
If the picture is 800x600, will the PS cut off the edges, or will there be black stripes on top and bottom (cover/contain)?
2. Is it normal if this picture is not on the page?