Need to change my rotors and breakpads, what determines the size of the rotors? I see that both 10.5" and 12.1" are available: https://www.hummerpartsguy.com/Brake...ors_c_993.html by measuring I reach 27 cm and 29 cm for rear and front respectively.

 

BRAKE rotors are replaced by simply matching up the diameter, height, bolt pattern. You can't go larger or smaller unless you replace the caliper, mount, pads, etc... Btw, your measurements are off @ 10-5/8" & 11-3/8".

 

Brake rotor sizes are determined by the vehicle's gross vehicle weight in which is either 10 800 or 12 100 lbs aka GVW

 

Where on the vehicle will the GVW be stated? Inside the driver door everything is overwritten with Norwegian rules and regulations.

According to the serial number this is class 3 in which could be 10 001 lb - 14 000 lb

From the manual: Rotor must be replaced if minimum thickness falls below 0.815 in. (20.7 mm). For 12,100 GVWR Hummers, minimum rotor thickness is 24.7 mm.
The ones for mine are 20mm wide, hence 10.3"