To find the appropriate board width, stand on a board that is flat on the ground. You can step into bindings or simply place your feet exactly the way you would have them when you snowboard. Your boots should be as flushed as possible over the edges of the snowboard. If your toes and heels are not right at the edge, you won’t be able to apply the proper pressure in order to make a turn. And, if your toes and/or heels hang too far over the edges, they will drag in the snow when you turn and result in many more falls than necessary and perhaps injury. This is called “toe and heel drag.”
In general, narrow snowboards are easier to maneuver and you will be able to make turns much faster with them. Wider snowboards are slower, more stable, and in general will respond well in deep snow.