There is not a fail-safe way to know which size of helmet to buy online. Ideally, you would be able to walk into our ChutingStar shop and try a bunch on, or be able to try on helmets at your local drop zone. But for some, it's just not possible.
Each manufacturer does post a size chart for its helmets. And that is a great starting point. You'll want to get a soft tape measure, which you can find in the sewing section of any grocery store. Measure around your head, level with your forehead and behind your ears.
Measure your head as shown above.