Sounds like you still have some confusion going on. First, always size before trimming. When the brass goes into the sizing die, as it first contacts the sides of the die, that narrows it. Narrowing it also squeezes it out longer. Running into the shoulder of the die shortens it again, but only at the shoulder, not at the neck. At this point the case is as long as it's going to get, which is why you trim only after sizing. An illustration I made is below.
If priming is lengthening your cases, then the primer must be sticking out. That is bad for ignition regularity. In a self-loader that increases the chance of a discharge occurring when it loads (slam fire), but it causes trouble in bolt rifles as well. A high primer isn't making firm contact with the bottom of the primer pocket. When the firing pin strikes it, the primer goes further into the pocket because it wasn't seated fully. This can cause failures to fire or weakened or delayed ignition of several 10's of milliseconds. Those give more time for any small movement of the gun to go off target before the bullet leaves, so it opens groups up. It can also cause more irregular muzzle velocity, which opens groups up at longer ranges.
Primers should be 0.003" to 0.005" below even with the case head. You can learn to feel for this with your finger.
