While there is no hard and fast rule, the rule-of-thumb is, never attempt to tackle a constrictor 9 feet long or over by yourself. They are far stronger than you might think.
I once grabbed an 8' redtail boa. My friend jumped on me (and it) to help. Best description looked like something out of the movie "Men In Black"
or something similar, with two grown people being whipped around and beat on the ground by a large, long muscle as thick as a man's thigh. We got it in the bag..... but it beat the hell out of us.
('Wouldn't do it again.)