Optimum signal gain chain

The optimum gain  should be: 

1. A)  large enough to allow the  smallest  input signal to be detected with sufficient resolution.
2. B) small enough so that the largest input signal does not overload the ADC.

Notice that using a larger gain value than required by  condition A may result in worse overall performance, since amplifying a signal more than required may result in larger swings and hence larger errors caused by non-linearities.  See also answer to this question.