I cannot find anywhere on the net what do these R0, R1, ... markings mean, sometimes they're referred as sizes, but then the detailed dimensions of an R1 and R2 screwdriver are listed as exactly the same.
Look at Table 1 in this reference, it gives the minimum and maximum dimensions for the square based on the screw sizes. The maximum allowed dimensions of the square are as follows (including my conversion to mm):
Drive Max dimension
0 - 0.0710 in 1.8 mm
1 - 0.0910 in 2.31 mm
2 - 0.1126 in 2.86 mm
3 - 0.1330 in 3.38 mm
4 - 0.1910 in 4.85 mm
The R0, R1, etc, refers to the bit or tip size. If you have Phillips head screw drivers you may notice the tips are different size, #0 being small, #2 being larger. Individual tips are usually 1/4" hex drive.