Head to a rental shop and the standard advice you’ll be given about choosing the right length snowboard is to “pick something that’s somewhere between your nose and your chin”.
While this is a good starting point, it’s pretty much the broadest possible guideline. Finding exactly the “right” length board for you depends on two factors – your height and your weight. However it’s also worth considering what snow conditions you’re most likely to encounter, and your terrain and style preferences.
As a rule of thumb, longer boards are faster and more stable but will be harder to turn (like a Cadillac!) whereas a shorter board will be lighter and more maneuverable but will offer less edge grip.