Currently, his regular collar is set at 20 inches. So his prong is probably at about 17 inches. Training collars fit differently, even martingales. Unfortunately, there really is no way around having to buy multiple collars along the way, especially for ones that are not unlimited for adjustment. There are some reasonable priced and I believe adjustable to a degree martingales available. I wouldn't bother with leather until she's at least a year. Even at going on 10 months I'm still having to adjust collars for size and I expect that to not end until he's at least 18 months, though it has slowed down tremendously, he is by no means full grown. Some don't really reach mature size until 3+ years.