In strong wind or in turbulent wind it can be necessary to add an extremely long tail to a kite, in weaker wind a shorter tail tail or even no tail may be best.
Tails add weight and therefore stability. The weight changes the balance, which also modifies stability.
Kites that twist and turn in turbulence will do it less with a tail because the heavy tail effectively says "this direction is down." The longer the tail the more weight it has that keeps the kite pointed the right direction.
Kites that are jittery or dart around will move more slowly with a tail because there is more mass and more drag. The heavier the tail, the slower the kite will move.
But kites that are struggling to stay in the air with insufficient winds should drop their tail, because the tail adds weight and drag.
As a result there is no simple math of how big a tail should be for all kites. It should be long enough to add stability, not so long that it drags the kite down, and that all depends on the conditions at the time.