most of mine got about twice as big...had to tie some down to train them to grow out instead of up...
depending on the strain, the avg i see is 12 to 24". A 6ft tall plant does not stretch to 12 or 18 feet. Sine most people start flowering when the plant is less than 2 ft tall, "double or triple" is actually fairly accurate.
i like how this kid tried to call out the GC coomunity so he could get an answer, instead of using the search option.
Id say double to triple. I usualy flp the lights @ 12 inch and end up with plants any where between 24 and 36 inches tall, Hope this helps