RMS = Root Mean Square. It is a measure of the average grain read from a microdensitometer across each density of a step wedge and then plotted against Log Exposure. Fine grain films are slow and coarse grain films are fast. Grain size may vary from 0.1 microns to 10 microns, but most films are a blend of 3 different sizes.
RMSG does not vary with frame size. RMSG is a constant with a given material, but the apparent grain goes down as frame size increases due to the magnification needed to view it.
RMS is not sharpness.