I have found in building hundreds of chastities for people I have never met, that most start out with a cb2000, cb3000, or cb6000. The devices' inside diemeter is 1 1/2" on all three. I have found that 1 1/2" fits 99.9% of the males on the planet as I have never had a complaint on this deminsion.
I decided to add other sizes to accomidate the larger and smaller sizes, just in case.
As far as length a standard mesuring tape does the trick.
The base ring is the tricky one. I have found that 1 3/4" is the small, 1 7/8" is the medium, and 2" is the large. unlike most makers if some one orders the wrong size it is no problem as I can resize the ring or make a new one. Some buyers even want all 3 sizes.
I am working with a customer now that I have sent just the ring to test the size and it is a wooping 2 5/8"!!!
Next is to gap between the chastity and the base ring. I have found that 3/8"
or a 1/4" gap fits most. Again if the gap is wrong I will have the customer send the device back to me and normaly with in a day or two it will be back in their hands.
I use priority mail for faster service. On adjustments all I ask is $10.00 to cover my shipping. I would like to see any other maker match that!
I hope I have answered all your question. For me the sale does not end after payment is received. It ends when the customer is happy. I am trying to build a brand and a reputation as the best chastity for the money.