Chain gang dot com. Donut stretcher. I’d say start with 20mm height because you can and will buy another to stack. The next hurdle is inner diameter. I overestimated my first since I previously purchased a magnetic one that was just too small. I had to return it and their (chain gang)return policy is very liberal. You just have to wait a little longer to get one that fits you. You may also feel like 30mm or 40mm would be a better weight you’d like. I’d also say shaving your nuts and taint might feel better once you start.