Currently, project thumbnails call for a ~5:3 ratio → 1920 x 1152 in order to not be cropped. This seems arbitrary.
With the advent of Figma Slides, now seems a good a time as any to make the standard into a traditional presentation ratio (1920 x 1080).
Hey @Ryan8, thanks for the feedback!
We’ll pass this onto the team for future consideration.
Great, delivered - but why has the cropping changed?
Before the update if there was a higher frame set as thumbnail, the crop area was like frame width x whatever the height was and cropped from frame top part (as it is also currently, that’s ok). Was perfect for website projects, because the hero section of the designed site’s homepage worked usually also as a great thumbnail, and you didn’t even had to design the thumbnail specifically.
Now the frame width is dropped and height seems to be fixed (about 6 times higher than previous outcome was). Why?! All messed up in my projects view, full of microscopic screenshots 
Please revert the full frame width rule. It would be bulletproof then with any scenario (both 16:9 and higher frames). Thanks!
Hey All, we officially updated file thumbnails for all file types to 16:9 (1920 x 1080).
@Silver - our team confirmed that this is expected due to the change (this is the result of trying to smartly fill the background color).
Please manually update your thumbnails to 16:9 ratio to make them fit to the right size.
Thank you and the team for implementing this idea! Excited.