Measuring how far the anus can stretch is vital to prevent tearing. So, to do this, you will need: lube, a tapered object and/or a partner. You also need a fabric measuring tape. You can do this sizing solo, but it is difficult to get a good measurement.
The way to do it is to rest on all fours or by leaning over something so you are comfortable. Then, your partner needs to apply plenty of lube to the anus and (the object they are using) or their fingers.
When you are relaxed, they can begin easing into your anus. It is vital not to let them go too far in. This can cause problems with your butt toy.
Once they reach the limit with several fingers or the tapered object, they need to mark how far they inserted. Once they pull their fingers or the object out, you can take the tape and wrap it around the fullest part where they have marked.
This will be the maximum your anus can stretch until you are accustomed to that size. If the manufacturer works in diameter, then divide this number by pi (3.14) to find the width/diameter using a calculator.