19" square,
A) compared to any reasonable person, this is still "low profile", and with TST or Rial wheels, nobody can tell they aren't 21s without a side by side comparison
B) Square allows for tire rotation
C) 19" is far more comfortable a ride, and far less susceptible to damage than anything larger, and performs as well or better than 20s or 21s when equipped with the same model of tires.