To add some extra details. The big five was not named because of their sizes, but because it was so dangerous to hunt them (also the reason that the hippo didn't make it onto the list and kills more humans in Africa than any other mammal). So the snakes that are most dangerous to "hunt":
Black mamba (Speed and potent venom)
Puff adder (Risk of stepping on it)
Gaboon adder (Also camouflage)
Cape cobras (speed and potent venom)
Afrock (Size and constricting ability)
If you take the same criteria used for the mammal big 5 then the boomslang, and some of the other venomous like vine snakes would be excluded based on their shyness and reluctance to bite.