You can ask this question to 10 different people and get 10 different answers. Reason being that there isn't a right answer. It's entirely based on personal preference.
If you are strictly a carver on the groomers, the longer the better I'm riding a board up to my forehead when I want to reach some high speeds. This will give you the stability you need at higher speeds. But if you are just starting, you may want to start off with something a little shorter and work your way up. Start with a size around your mouth or chin. This will give you additional maneuverability and allow you to learn how to connect your turns.
If you would rather ride in the park, you want something much smaller. A park kid wants a board up to the bottom of their neck. More companies are making their boards designed to ride 10 cm shorter than what you are use to. This has its advantages as well as disadvantages. A shorter board will loose some stability no matter what they tell you.
As a parents buying for your kids, I would advise you to buy a board around their nose. This will usually give them about 2 years of growing room before they may need a new board. Most parents tend to want to buy a board with growing room but don't over do it. The nose is fine. If you give them a board that is too long, they will usually get too frustrated and not associate snowboarding as a good experience.