To see if your kite requires a tail, fly your kite without the tail in Low/moderate wind condition. If the kite flies straight, without wobbling or incline to either one direction than you do not need to add a tail. Now try flying it in strong wind, or you can run with it. If it starts to spin or roll or any of the above this is your hint to adding a Tail. Tails create a downward drag which in turn makes the kite stable. Tails are major help in strong stormy winds.
Low wind = Smaller tail
Strong wind = Longer tail
Tails are not always used to stabilize the kite, designers also add them for creativity. It adds color and movement in the sky. Also makes the kite look big.