How much is grain per detail (and tonal smoothness) a function of format size?

First, the film grain per area is a constant of format size. This is important.

We can estimate grain increase per one stop increase in required film sensitivity as ~2x. In other words, if you shoot a film at ISO 200 then there will be ~2x more grain per detail than the same shot at ISO 100.

Obviously this is a very rough approximation and a rather pessimistic one at that....

keithwms