Not how big, more like if there is branches/top/material to take a clone.... Doesn´t have to be big at all, but the bigger the clone part is the more probability for success (due to nutrition it holds within herself when cut).
I have found small trimmed clones root better, less transpiration or water loss. I can take clones off a plant 6" tall and that plant was a clone off a mature mother plant. When I top a plant I don't waste the material, I'll use a clone 2" tall and it puts out roots and grows and flowers normally. Just ask Freezerboy.
I have no reason to doubt you I guess small/new growth taken as clones will compensate with chem reactions to grow, my point is that a bigger will have nutrition to grow before having roots, and therefore may last longer before dying.
But since I´m still failing 60% of the time I should keep my mouth shut
But what I wrote is a general opinion/technique with other plants.....