hey, erin! :--)

the difference between a harness (front-clipped, not leash clipped to the rear-ring) and a headcollar of any brand, is that the harness can go on the dog immediately and be used; the headcollar OTOH requires some habituation. dogs who have never worn one need to grow accustomed to the funny feeling of a strap across their nose.

i use the Gentle Leader by Premier-Pets, it adjusts on both nose-loop and neck-loop for a good fit. FIT is crucial - too loose on the neck, the nose-loop will be too far down the face, and it will have to be uncomfortably tightened to stay on their face!
that is *wrong*. the more-snug + higher the neck-loop,
the more-open the nose-loop can be. that is the correct fit.

i will post specifically about fitting + habituating the GL elsewhere -
look for the thread, if U would like the details. cheers,
--- terry

terry pride, APDT-Aus, apdt#1827, CVA, IPDTA, TDF