Got a measuring tape or ruler? That'll work.
Get measuring instructions. If you don't have a measuring tape, a piece of paper and a regular ruler will do the trick.
When you know your length and circumference, enter them here. There's your size code.
Why do we use mm instead of inches? The condom production process uses mm, and it tends to be a little more accurate. Here's an inches to mm converter to help you out just in case: