Think there a factors ... which depend upon how site is setup and maintained and used ... the latter perhaps the most important.
Case in point ... moodledata/temp/backup/ is used for building backups of courses. Creates a contenthash named directory, then uses that directory to build a backup. At the tail end of that routine, copy is used to move the resultant .mbz file to destination as set in preferences or via command line script. So for a course backup that is 139Gig, for a little while there, there is really double to triple that ... all the files folders used to build it still there ... the .mbz file itself ... and the file being copied in whatever destination.
Granted your site might not have courses that size, but do you know the courses whose backups would be the largest and what the size of that backup is?
Automated backups would do one at a time.
Have any teachers who are practicing good habits by backing up their own courses right after a major assignment due date or major test?
Anything else or any behavior of Moodle that would use the moodledata/temp/ area comes into play ... how about annoation of uploaded assignments? (if using that) ... document conversions so teachers can annotate/grade.
So whatever one sets the ramdisk to had better be able to handle the heaviest usage of whatever is put into ramdisk.
I'll be totally honest ... I've never tried to figure that out or calculate that. So, if going forward, I would think dynamic setup might cover your arse! .... and hope that it does! 
2 cent opinion!
'spirit of sharing', Ken
