Internal impedance of Class AB amplifier?
Finding the peak input voltage that still results in linear output drive is not too hard.

Think of how either Rl is projected back to the base of the transistor, or how Rb is projected to the emitter.  You should be able to do that knowing the transistor gain.  After that, it's a voltage divider to find what either the drop on Rl or Rb are.  Remember that the B-E junction takes away a little from the overal available voltage.  Then accounting for the 700 mV B-E drop, you get one from the other.

Once you know the maximum base voltage, you note that everything above applies to the other side, and the P-P voltage is twice the maximum.