Facebook

What Dimensions
Cover photo 851 x 315
Display picture 168 x 168 ?
Highlighted image 1200 x 717 (appears 843 x 504)
Share link (og:image) 940 x 492 (1.91:1, appears as 470 x 246)
Share link (square) 1:1, appears as 114 x 114?

Twitter

What Dimensions
Page header 1500 x 500
Display picture 400 x 400 (shown as 200x200)
In-stream photo preview 440 x 220 (2:1)

References

  • https://developers.facebook.com/docs/plugins/checklist/