Cylindrical Rings need the most accurate size to work.
You must use a caliper (preferably digital caliper) and take the width to the nearest 0.01mm at the widest point of the thumb. If you use a non digital caliper, please check here and learn how to read it:
Measure three times, first a loose measure just lightly touching the skin, and a tight measure pressing against the bone that it even hurts a little. Last, take the length of the proximal phalanges of the thumb.
For example, this tight width reads 21.0mm
This loose width reads 21.8mm
Lastly take the length and it is 22mm
Measure the thumb's accurate circumference at the first joint below the finger tip.