Thank you @panaghiotisadam. Is this a temporary thing or is it the new standard? If it's the new standard: do i have to manually tweak every image I've posted on my blog in order for them to appear again in the single post view? Or do I wait for an automatic script, patch or something?
So far I've made a few tests and it works fine. I've found two ways to solve this problem: 1) create the image again with the new size (640 x 392), upload it as the new featured image in the post, delete the old one and update the post, or 2) find the image in the library, update it, scale it, write 640 instead of 620 (a red exclamation sign appears) and save it. Both worked. The first one is huge work considering hundreds of images (plus embedded info). The second one is much faster but compromises quality.
And just out of curiosity, where did you learn about this? And why would someone do something like this if he knows so many people will be affected unwillingly?
Maybe nobody knows this but I'd like to hear your opinion: let's say I take the time to edit every single image in my blog so they appear again. Maybe next year they impose a new standard size and I loose everything again?