Why does the B-52 have such a tiny rudder?
There are two reasons and they date back to the early designs for the aircraft. Keep in mind the tail is different now than when it was originally designed. Boeing has originally planned for an all moving vertical stabilizer but engineers doubted its reliability and the systems to operate it simply would not fit in the tail at the time. More importantly the original designs had sufficiently larger control tabs but this lead to an elevators and a rudder that had a lower critical mach number than the wing (i.e. the empennage would stall first). The engineers shortened everything to create a more stable situation as is noted in this passage: 


  Concerning the rudder, the situation is much worse. The B-52 rudder
  and elevator have an exceptionally narrow chord. Most airplanes have
  at least a 25% chord, which allows sufficient power and effectiveness
  in both yaw and pitch during takeoff, landing, and asymmetric
  conditions. The same is not true of the B-52 with its 10% chord rudder
  and elevator. The chord was reduced because original designs of the
  aircraft had the tail exceeding critical Mach prior to the wing. The
  chord of the tail was reduced to ensure that the wing reached critical
  Mach first.[15]
  
  The original designers knew that the rudder was underpowered. An
  aircraft the size of the B‑52 requires a massive rudder that does not
  exist. This rudder is insufficient for the basic needs of the
  aircraft, let alone for asymmetric engine-out scenarios. Original
  plans called for an all moving vertical tail, the first of its kind.
  However, Boeing doubted the reliability of the hydraulic actuators
  necessary to achieve this. The design was also too heavy for the tail
  structure to sustain. Therefore, Boeing designed, built, and
  incorporated the yaw-adjustable cross-wind landing gear,[16] what is
  known to B‑52 aviators as ‘cross-wind crab.’ This system allows the
  B-52 to operate during normal conditions of take-off and landing in
  crosswinds. However, the true problem was asymmetry.