Here's a sizing guide to help you with your order:
Size 1: 5 - 10 Pound Dogs
Size 2: 11 - 25 Pound Dogs
Size 2 Thin: 11 - 25 Pound Dogs with short noses
Size 3: 26 - 55 Pound Dogs
Size 4: 56 - 80 Pound Dogs
Size 5: 81 - 110 Pound Dogs
Size 6: 11- 150 Pound Dogs
Size 6+: Dogs over 150 Pounds
If you have a large or highly reactive dog, I suggest you purchase a Freedom Harness along with a Comfort Trainer and Double-Clip Leash. The pairing of these training tools will give you the ultimate control to redirect your dog quickly and gently. Should you be uncertain of how to use any of these training tools, purchase the book, Click and Easy: Clicker Training for Dogs, as it details how to use head halters and front connecting harnesses individually, and together.