Not sure about NJ laws, but does it have to be any size/shape? Reason I ask is my gf and I both have luminescant Yamaha Tuning Forks stickers on the back of our helmets.
After a buddy 2 up ride last night, my friend and his wife commented on how amazingly bright those stickers are. They said they just shine and shine from far away and close. These are 4 1/2" diameter. I got a discount, but I think the stickers ran about 4-6 bucks retail each. They ride an R6 and my girlfriend ordered them a set today when she went in to work.