LEGO stormtrooper pauldron dimensions
Based on this information from Brick Owl, the Imperial pauldron would fit in a rectangle of dimensions 2.7 x 2.3 centimeters. The center hole on all capes must be able to clear the neck diameter of approximately 4.88 mm as was quoted here backed up by my own measurements.

If you are looking to make a pauldron for yourself, you might consider finding a trace of the desired pauldron and adjusting the scale until the image is 2.7 cm at the widest. Making a 5 mm hole will suffice so the piece does not fall too low on the shoulders. Hope this helps.