Michael
Technique is more important than size. Learn to use what you've got.
What camera do you have ? More important, really, than maximum extension, is how secure the camera is. For instance, compare a 10" lens, a 12", an a 14".
A 10" lens, from 2 meters, needs 11 1/2" extension, and makes a full head (12") about 1.8" high on the film.
A 12" lens, from 2 meters, needs 13.8", and the head size is about 2 ".
A 14" lens, from 2 meters, needs 17.3", and you can't do it.
From 2.5 meters, you need 16 .5", and you get about a 2" head size.
I'd use the 12" in this case, for it would surely be more stable than the 14".
Whatever the choice, practise before loading up on gear.
8x10 portraiture is a different world than 120 portraiture.
s