[help] peak-to-peak value in flash adc

when designing a gps receiver, a simple rule to decide the overall gain is to amplify the signal to the maximum range of the ADC. but i am wandering what determine the maximum range of the ADC. A paper mentioned the peak-to-peak value of a 1-bit adc is 100mv,what is the reason? if i design a 4-bit flash adc in 0.25um TSMC techology, what is the proper peak-to-peak value? thanks a lot!