What’s the best content image size if you want to share your site on Facebook?
Nothing less than 200 x 200 pixels
First of all, it’s worth knowing that images smaller than 200 x 200 are discarded by Facebook, they are too small!
Images smaller then 600 x 315 are ok
Images between 200 x 200 and 600 x 315 are ok but Facebook will show them in a small, unattractive format, see the following image, would you click on it?
Imager larger than 1200 x 603 are GREAT!
Images larger then 600 x 315 pixels are displayed with the large format, however you must use images of at least 1200 x 603 pixels for the best results on high resolution devices.
Beware that larger images are heavier (in bytes) so I’d suggest not to use images too large, even if they can render nicely.
Last but not least, try to keep your images at an aspect ratio close to 1.91:1.