title_body: SVG covers: which size must be used?
upvoted_answer: Since you can only specify one size, and not have the selected image depend on the devices resolution, you best take something that is as large as the largest expected screen format. I would at least go for 758 × 1024, but you can consult this table and decide for yourself.
  
  Scaling down is normally OK, whereas scaling up always introduces artefacts.