Low-voltage limit of quantizer such as ADC
With a sigma delta converter, which is a 1 bit ADC, with noise shaping feedback, and DSP filtering, you can achieve any arbitrary resolution. 

That means your actual usable accuracy, and repeatability, are limited by noise, and long term drift. Once you have a few LSBs of noise, or of drift from minute to minute, then there is little point in increasing the resolution further.

If you measure in a 1Hz bandwidth, then even in theory, at room temperature, your noise power will be at least -174dBm. In practice, it will be much, much more. You can convert from noise power to noise voltage through the input impedance of your chosen 1 bit ADC.