Boy, I can tell stories of my searches of head halters *G*.
I have a Gentle Leader for Gaci, which fits very well. She used to be terrified of it, and I had put it away for a year and a half, but eventually I brought it back out and, after reconditioning her to it, she takes to it much better than she used to.
When we first moved to the city, Shimmer found the environment very stimulating and until I could work to teach walking nicely on a leash, I needed something for management. So knowing how well it worked for Gaci, I went out and bought another GL. Bad luck. It didn't fit her head right at all. If she pulled into the GL, the nose strap would actually pull back and cause her to squint, and rub up under her eye. It was fitted properly, but it didn't fit her nose right, and it wanted to move around on her neck, even though it was tight as it needed to be. I tried both the small and the medium, and both were not working. I bought a small halti (Size 1), and the neck strap was HUGE! I couldn't make it small enough to fit her neck. They have ordered a Size 0 (yorkie size?), but in the meantime I found another brand name called Avenue, which is basically identical to a Halti, except the neck strap seems to be more secure. But it has the construction like the halti with the extra strap, and it doesn't ride up if she happens to pull, and she took to it much better than the GL.
I have found they work well, but they don't tend to be a permanent fix. Ify ou aren't teaching with it, dogs CAN learn to pull against the GL by positioning their head and shoulders just right. It works best when working with teaching programs.