Snowboarders are people, too! Sizing boards is similar to sizing skis but with more of an emphasis on the weight, as well as the height, of the rider.
For adults new to snowboarding we place one end of the board on the floor by their feet and measure to around their chin. For more experienced riders, we recommend a longer board, measured to the nose or even forehead for length. As I said before, more consideration should be placed on the weight of the rider for board selection. I.e., the heavier or more aggessive the rider, the bigger the board they will need.
For children learning to snowboard, we might measure even shorter - somewhere between their chest and chin.
Remember, boards come in incremental sizes. If you're in between two of them, measure down for beginners and up for more advanced snowboarders.