Most of the 8x10 PEOPLE shooters I've known liked the 18 - 19" with 8x10 for a head and shoulders picture. I like that myself. A headsize of 2 1/2 to 4 inches is quite attractive.
With a 14", lens, I think a good full length shot that has a figure about 6" high. Get closer, it becomes bizarre.
Shooting at the same distance as I would with an 8x10, the 18 - 19 is very flattering to the subject, and the bigger neg adds a pleasant amount of context to the picture. Of course, the head size is the same size it would be on 8x10.
Shooting tighter, with a 24" lens, the headsize looks nice 4 to 7 inches. Bigger, and it gets a little creepy to me. Not to mention, difficult to work with.
I think the camera and tripod will determine whether you can efficiently shoot larger than a 3" head size with an 18" lens, and whether you can shoot a 24 at all. A saltzman, majestic, #5 Gitzo or Foba is pretty much a requirement. Racking out a 24" lens on an 11x14 Deardorff (V 11) is safe. A willing sitter, 3000 - 6000 ws of flash are also nice. For shooting in available light, which is a lot of fun, and makes very pretty images, a big studio stand makes life worth living.