Don't be tempted to let a sailing buddy teach you. Such lessons usually do harm to both learning to sail and to the relationship. A friend who is a high wind 'short board' expert also might not be a good instructor since he or she probably has forgotten basic sailing rudiments completely. If you picked a good windsurfing school, they will provide you with an appropriate board to learn on. Pick a board with enough flotation to allow you to stand and balance easily.
Kids can use shorter boards.
The amount of flotation you need depends mostly on your weight. For example, if you are under 160 lb. If you are between 160 lb. There are several misconceptions about sail size. Some think that bigger is definitely more macho and will make you go faster, while smaller is necessarily easier.