Sheaffer used a dial indicator nib gauge for measuring nib sizes. The nib was inserted into the gauge, and the size read off of the dial. A given size being nibs that fell within a given range. What is listed below were the ranges given on a gauge that I saw in the Sheaffer service center prior to being closed in March 2008.
Measurements are in thousandths of an inch. MM in bold type.
XXF = 0.010 - 0.013 = 0.254mm - 0.33mm
XF = 0.013 - 0.018 = 0.33mm -0.46mm
F = 0.018 - 0.025 = 0.46mm - 0.63mm
M = 0.025 - 0.031 = 0.63mm - 0.79mm
Broad* = 0.031 - 0.050 = 0.79mm - 1.27mm
Stub = 0.038 - 0.050 = 0.97mm - 1.27mm
*there was some overlap on the gauge. The broad nib is finer than the stub nib at the low end of its range, while the high ends match.
updated 5/16/19 to show conversion to metric units. Sheaffer's original dimensions were in thousandths of an inch, but most nib sizes today are graded in mm, so this makes comparison to current standards easier.