Yeah, you can use either a string or a drinking straw to measure her mouth then you need to add 1/2 inch for the correct size. As for how tight it needs to be in her mouth, I don't go by counting wrinkles because every horse is different. I will adjust it to where it is comfortable to the horse. If they are fighting the bit, then it is not fitting them right. I put it where it looks right then ride them for a while. If they don't feel right, I either take it up a hole or let it out (depending on how they feel in my hands). I would not suggest a mechanical hackamore on a young horse. They have the potential to be incredibly harsh and cues can be confusing in them. If you like, you could get a bosal but you can do the same training in a rope halter that you do in the bosal. I would suggest that you get a bit that fits and go from there.