I do not think that one can give a single number here. The bottom line is that one needs oxygen to get oxidation. At the same time, pitting corrosion will only occur if there is an electrolyte present. The crevice that will result in "crevice corrosion" is therefore a function of the amount of O2 in the vicinity, the type of electrolyte present, the presence of microbes that tend to exclude oxygen but include electrolyte, the presence of deposits or sediments and the size of the crevice. (Probably some more variables that I have forgotten)

As an example, most flanged joints (Very tight crevice) do not experience crevice corrosion for the simple reason that the electrolyte as well as the O2 is excluded.

Above is the practicallity of the situation. Theoretically, the minimum size would be that through which O2 molecules could no longer fit. (Smaller than you could possibly have dimensional control over.)

Niekie Jooste