I understand pretty well how an op-amp operates at small signal. You need to bias the op-amp in order to set the transistors at their constant current region, to achieve max output voltage swing, and to eliminate DC voltage/current at output. Once these are set, you can calculate the small-signal gain of the amplifier. However, how do you know what is the large-signal gain of the amplifier? Moreover, how can you tell the DC input and output resistances of the amplifier?