For me there are a couple of things to look for on the size of the stab fences. One is a spin entry. If the model will not stay on heading through the stall entry before you kick the rudder you need more or larger fences. Usually this is more prevalent with an upright spin entry. Models will usually stay stable in an inverted entry. The other thing is inside corners. If the model seems to track well in large inside maneuvers but is inconsistent during inside corners, you need more area. Most models will track well through outside loops and corners but if they are shy on fin area and it's placement they will seem to wander during insides.
When you get this right the model will feel more locked in at all times and it will also improve in turbulence.