Maximum Vpp that doesn't change the output offset
As long as the average input voltage does not change, an ideal amplifier will produce the same average output voltage. The average output voltage thus changes if the non-ideal nature of the actual amplifier kicks in, and does so in a way that affects the negative and the positive part of the square wave differently.

In your exercise, exceeding the slew rate or the gain-bandwidth product will deform the upper and the lower part of the square wave the same way and not affect the average. But if the amplifier clips the positive side of the wave at +13.5V, it still has enough headroom on the negative side, due to the large offset voltage. 

I hope this answer is able to give you an idea where to start with the task.