When you plan to buy the male chastity device from lockthecock.com you need to be sure that the measurements are right. The main ring measurement has to be perfect. This is the ring that will circle the penis and the testicles. If the fit is incorrect then it could lead to lots of problems. If the ring is oversized then the cage can slip down or it can also fall off totally. If the size of the ring is small then this will cause pain and discomfort. This may also lead to the blood supply getting cut off to the genitals. The best fit is one that fits perfectly and avoids slipping. It should be loose such that the blood flow is not stopped and you feel comfortable to wear it.
In case your testicles turn blue or purple then the ring is very tight and you need to change it. It may be really difficult to get a chastity device that fits perfectly. This is especially true if you plan to measure it with a tape. You could instead use a string around the genitals and measure it. But the best way is to visit a sex shop which has instruments that let you measure the dimensions accurately.
Be careful that the string that you use for measurement is made using solid material and not stretchy materials. This will give inaccurate results. Also, make sure that the string used is smooth so that you do not feel any sort of irritation. Start with the large size and gradually go lower. Settle for another one that fits properly and does not cause any pain when worn.
Also important is to measure the length of the cage. This is important because you want the cage to belong such that the penis tip comes to rest at the cages end. A small cage would feel like the penis is held in the body and a large one will make the urine splash.