Slim faces

I'd like to hear your experiences of producing flattering head shots of people with long, slim faces.

Which focal length do you find produces best results to add a little fullness without enlarging nose/tucking ears away?

Slim, fat, rounded squared flat long whatever people are how they are... Why you want to change them?
IMHO i won't do the change on purpose,
I would be myself a bit upset if made on purpose (unless it s a joke which is different)

After a certain point, lighting is more important than focal length.

Broad lighting may help add some fullness to a slim face, and short lighting may help lengthen and slim a rounder face. But the shape of the nose also plays a big part in it; you might not want to use broad lighting on a subject with a big pointed nose.

its all about the lighting.