i think there are to many factors to follow one strict rule.. i know people who swear by everything from a 6-10 foot rule and i see to many things that vary like for instance

1)species.. a python is obviously alot stronger then a colubrid but both can make 10 ft lol
2)temperment of said snake, some snakes are mean and need others incase they do get the better, my tic is probly goin on 7.5 ft and i handle him alone and control him with one arm and im only 5'6 maby 135lbs.
3)level of comfort, i have a level of comfort with my tic because iv had him since he was 3ft and have got to know his attitudes and moods and know when i shouldent be handling him period
4) how you handle it, as said a 6ft boa killed a man because it wrapped him neck.. i Never let my tic get a coil on my throat i let him drape over my neck but never around my throat simple because of how strong he is should something set him off.
5)how well you know said snake/what you know about the species.. untill a few days ago i thought scrubs were gonna be my next snake but have seen what a 10ft female can do and with there attitude iv decided ima wait till i have my own house and a person who is around more often.

i mean these are just a few oth the things i take into account when working with snakes and try not to set a *rule* persay but more what i can answer with these questions. plus i mean in all reality like a previous poster said a 7ft carpet can be handled by a 10 year old with ease when u say 6ft snake for the most part its still hanging around your arm without much trouble (not taking giants into account really burms inperticular)