Surely there is a bearing and seal supply house somewhere near you in MdR. If you have the old O-ring, take it to such a place and they will either have one in stock or have the gauge to determine the size correctly. Buy several... they will be really cheap
... put the spares in a Zip lock with a label as to what it fits. You will be glad a few years from now when you need another one!
If you don't have the old one, take the transducer. The folks behind the counter are much better at determining the right diameter and cross section that we amateurs are.