1 - Know your height and weight and find the correspondent snowboard size.
2 - Tune that snowboard size for your riding style and level: beginner -3cm | jibing/buttering -5cm | all-mountain freestyle -3cm | all-mountain freeride +3cm | backcountry/powder freeride +5cm | trees > short and wide
3 - Because one cm up or down wont make a difference if you get, for example, 156, go with 155, 156 or 157. Done.