What does R0/R1/R2/R3 stand for when referring to Robertson square screwdriver?
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