Bob is right about the "Official Measurer" on the back of the vests (provided by Bernie Conway, I believe). This vest marking, even though small, pretty much solves the problem from behind.
Usually the scolding from bikers is from bikers approaching from the front. I've been trying to come up with a similar warning marking for a vest, readable from the front when the wearer is bent over, pedaling. So far, I'm stumped.
The sign on the bike might be the answer. I certainly don't care about aerodynamics while measuring. A bright yellow sign the size of a half sheet of paper (5.5 high x 8.5 wide) might work. A little pouch on the back might be handy as well.
Using the corrugated plastic that election signs are made of might be just the thing. I have a sign company owner friend who might be able to help. I'll keep posting here if I make any progress.