In most cases the shorter versions of a plane need a bigger fin, A318 for example have a bigger fin than the bigger brothers. Short plane means less stability and thus need a larger fin. I cannot see that the ATR-72 has a undersized fin or rudder. Max demonstrated cross wind on dry runway its 35 kt for the ATR 72-600 and that indicate that it has a very good rudder.
The only reason for the rudder is to "overcome the adverse yaw, created by the down turned aileron when making a turn". That is the basic reason for a rudder, reason, simply put, if you start a left turn, the nose of the aircraft, without the use of rudder, will turn right first and only then will it come back to left as you want! If climbing, which these guys were doing at the time, non use of the rudder would increase the drag on the aircraft because it is now slipping in the turn, which also raises the stalling speed to a higher than normal value, which could have caused the left turn which this aircraft did prior to losing control alto gather. Now there are other functions for the rudder in addition to that, one being, overcoming the adverse yaw created by a failed engine and the torque and "P" factor now created by the still operating good engine! Last, but not least, controlling yaw about the vertical axis of the aircraft in all flight realms.
When teaching basic flight instructions to students, one good lesson for them is to teach them to "push the high rudder pedal to correct the low wing condition, if one, example, in a stall, if the aircraft drops off into a low left wing down conf, then use the right rudder to correct the low wing condition, of course using aileron to assist you in getting back to wings level attitude. Why is that important, because the stalling speed of the aircraft is higher in a turn than in level flight, so no matter what is happening, you want to maintain a wings level attitude until you regain flying speed. And to, if you don't get the nose down or power restored, you are just going to stall again. Also, when stalling an aircraft, your nose is going to be pointing down, but in a wings level attitude, you will regain flying speed much quicker than in a turn!.
Again I make the argument that if the ATR 72 had a larger rudder, then it could operate at most slower speeds than it does now!
