Hi Frank,
I was hesitant to reply as I can't find the confirmation of the size I used, but since you haven't got a response yet I thought I'd share my reasoning.
I based the markings on my halftrack off of a period photo of an M3A1 attached to the 4th AD. Based on a 20" star, and the slightly smaller "8" placed aft of it on the side armor, I made mine 15". I do remember confirming this measurement at the time and before I ordered my stencil, but for the life of me I can't find that info.
Certainly not the definitive answer I'm sure you were looking for, but maybe it'll get the conversation started.
-Tom