It depends. The ISO media container format (.mp4) uses strusture elements called atoms . It depends on the number of tracks, H.264 encoding properties, multiplexers also want to put their names in a file in the atom free.
Expect it to be at least 1 kb. Something over 4k is not very likely.
If you come across a line mdat(the title of the atom of the movie), the title of the file is possible .