Bump, I was about to post the same question.
Is this not possible with Strapi? I would have expected the media field to have an option to specify a fixed or minimum resolution for uploads. Imagine the frontend needs to show a big fullscreen image and a technically inexperienced author uploads a 100x100px image.
It also seems not possible to at least add a note to the field what resolution should be provided.
How do you guys handle this?