They have a size chart if you scroll down, they tell you how many ounces for each size. The smallest weight they have is 10oz but that's way too tight. If your trying to find the right size I'll give some pointers on my experience. The 40mm ID hole is the biggest they have, but it's a bit wide for me and others with normal sized balls.
There's many ways a 40mm ID weight can slip off. It could fall off if you have more weights stacked above it. When you sleep your balls can retract and go into the ring, and it will most likely fall off when you use the
Bathmate. That's why I think the 36mm ID will be perfect, 35.9mm was the best average measurement if your using variables like the secret leather measuring guide did.
They found out that 35.9 was in the range for perfect ball stretching. I think the 36mm x 20mm will be a good starting weight. Then I can stack 40mm x 20mm weights on top because the balls will need all the space they can get for comfort. As long as you have the small inner diameter weight at the bottom your fine and the 40mm will be more comfortable on top. People with farm eggs like DLD are ok with getting all 40mm's or maybe a 38mm at the bottom to be safe.
If you get them from that site they give you a choice of 20mm height or 30mm. I've read that the shorter weights are more comfortable and they act as a spinal column. For example if you stacked four 20mm rings the total height on your balls would be 80mm.
If you stacked three 30mm rings the height would be 90mm. So you can picture in your head the 20mm weights would move more freely and act as a spinal column. The tall 30mm's will just be a big hulking mass of weights on your balls. I'm really looking forward to wearing this weight while in the
SG and
Bathmate to keep my balls from retracting. Hope this helps if your trying to size up, any Qs please ask.