Mondo size is simply the length of the boot's internal footbed measured in centimeters. It's the most accurate way to size snowboard boots because it's based on an actual physical measurement rather than inconsistent street shoe sizes. To find your Mondo size, measure your bare foot length in centimeters on a Brannock device or ruler, from heel to longest toe.
When trying on boots, they should feel very snug—your toes should just brush the front when standing straight, then pull back slightly when you bend your knees into a riding position. This feels counterintuitive, but snowboard boots pack out (stretch and compress) by a half to full size over your first 10–15 days of riding. If they feel comfortable in the store, they'll likely end up sloppy and unsupportive after break-in.
Always size by your Mondo measurement, not your street shoe size, and account for that pack-out when deciding how snug to go.