NVSRAM - Storage Capacitor Selection
The datasheet says that the capacitor needs to provide enough energy for the "store to nvram" action to work. Cypress will have done simulations of the chip to work out how much current is consumed for how long during this phase. That gives a value in coulombs for the charge consumed in that time. Plus their knowledge of what the minimum voltage must be during this process, gives a value for the capacitor.