Without a doubt, rudder is the largest piece on most 3D planes.

Also, it's most likely a tail dragger w/ long tail moment, which means a lot of load goes to rudder during landing, taxing, and evening just starting the engine. For this reason, I use metal gear on rudder for its structural strength on planes heavier than 7 lb.

If 1 servo drives 2 elevator pieces, then I would put my a strong servo there too. It's better to get more strong servos than to end up with 1 less plane.