A common starting point is that your board should reach somewhere between your chin and your nose when you stand it on end next to you. But that's just a rough guide—your weight actually matters more than your height. Always check the manufacturer's recommended weight range for any board you're considering, because a board that's too short for your weight will feel unstable and wash out, while one that's too long will feel sluggish and hard to control.
Shorter boards are easier to maneuver and spin, making them great for park riding and quick turns. Longer boards give you more stability at speed and better float in deep snow, which is why freeriders and powder chasers tend to go longer. If you're a beginner or mostly ride groomers, something in the middle of your range is usually the safest bet.
Don't forget that shape and width also play a role—a wide, short powder board can float just as well as a longer narrow one. When in doubt, size based on weight first, then tweak based on your riding style.