I show a gravatr image of users in my site. How can I know the best highest resolution to use? e.g. which parameter "s" should be.
https://secure.gravatar.com/avatar/?s=250
of course it depends in the user image, but gravatr must know the resolution of the initial image and can advise me on best highest size.
It seems the maximum size changed drastically in the last 9 month
You may request images anywhere from 1px up to 2048px, however note that many users have lower resolution images, so requesting larger sizes may result in pixelation/low-quality images.
quoted from the Gravatar image Request tutorial