Video Uploads in a Hostile Legal Environment

I am prompted to start this thread by reading about the new ATS upload utility, currently in beta form for images no larger than 500K. It is a question that has been on my mind for quite a while. I know ATS has issued directions on this topic, but I'm still not sure if the directions cover all situations one might encounter.

I'm talking about uploading video clips from videos made by or about people and events where there is a highly charged and aggressively litigious stance adopted by one or more parties involved in the situation and where there is a very small chance of getting permission from anyone involved to excerpt parts of the video.

I want to know if there is a recognized size of clip that, under fair use laws, that one may upload, without permission, without fear of legal sanction, regardless of the issue under discussion.