How do you find the right size snowboard?

When you stand against it, it should be between your chin and start of your nose.
They say a snowboard should measure in between your chin and nose standing up. Example a 6 foot tall person should by a board that measures between a 156-159
The best way to determine the size of board you need is by your weight. Just google snowboard weight sizing chart and it should be fairly easy to find one.
My son is 5 foot 3 inches and weighs 105 lbs what size snowboard should i buy
It depends on your height. When you are renting a snowboard, the employee will measure your heigth and then give you the snow board that fits you the best. If you are buying a snowboard they do the same thing.