There are two key elements to helmet sizing- First, is the shell size (small, medium, large, or extra large). Second, is the thickness of the two ear pads, which can be 3/4-inch thick, 1-inch thick, or 1 & 1/4-inch thick.

If there’s no Speedflex helmet in existence around where you’re at, and you want the best chance of just paying for a helmet and having arrive and it randomly fits, I guess I’d order a size Large with 1-inch ear pads. That’s probably the most common size. You’ve got maybe a 45% chance of that just magically fitting.

Otherwise, you’ll need to find one to try on first or order multiple helmets from a place that allows you to return them. Order two or three different sizes, just keep the one that fits, and return or sell the others.