Both the 3.5 and 3.6 documentation say "If this is set to zero, the check is disabled and any size may be edited." But, this doesn't work.
It also goes on to say "The default value of max_file_size is determined by the global control body setting whose default value is 100k."
In examining courses source, for 3.5.3 (what I'm running) and 3.6.1.
It appears that if the attribute is undefined or 0 (when determining the edit_defaults), it uses the EDITFILESIZE global. Which is 10000 (10k) in cf3globals.c...though cf-agent's initialization sets it to be 100000. Probably, would only be an issue if cf-promises did filesize checking/warning....
In files_lib.c, it only does the size check if the value is not 0... which is promised by the previous function.