What is the gain of this capacitance measurement circuit?
If you pick Rf to have a high resistance relative to the impedance of Cf at the input frequency, the gain before the peak detector will be approximately -Cs/Cf. 

That's a consequence of the feedback impedance being XCf || Rf ~= XCf and the normal equation for the closed-loop gain of an inverting op-amp amplifier. 

One would normally pick the resistor in such a way- typically the purpose of Rf is to provide a path for the bias current of the op-amp. At very low frequencies the gain will be less as the resistor influence manifests, and you can easily write an equation for the gain including the resistor and including the op-amp open-loop gain too, if you want.