@maxim can you share images that show what low roughness and normal roughness look like (zoomed in to the grain preferably) I think this might be a good replacement for the grain size in Color.io (which I don't think is particularly nice or useful) Thanks!

Sure! Btw, in Lightroom, "Size" is another slide, so they are not interchangeable. I suppose, talking in Photoshop's Noise filter terms, Roughness = Amount of noise (variability of noise pattern, when it is low, it looks like uniform noise), Strength = Ievel of contrast of the noise layer, Size = gaussian blur of the noise layer