how can I calculate the smallest measurable voltage for a given adc?
Some ADCs are nice enough to define the number of effective bits they have, which is the number of bits that are above their noise floor. You'll often find for instance that a 16 bit ADC only has 14.5 or 15 effective bits. 

Yours doesn't, but it does give an input noise of 50nV RMS (or about 18 effective bits), so values smaller than that will not be measureable without at least averaging. 50uV should be measureable, at least assuming you obtain the ideal performance listed in the spreadsheet.