I have a really large noggin, and have tried on all kinds of helmets. When I tried them on, the XXL Leedoms and Boeris fit, but were snug and uncomfortable. Plus they were kinda heavy. The best fit/size/weight lids I found were the XXL Giro nine.9 and Fuse, with the Fuse being a little too big.
Oh, and if I buy a fitted baseball cap I look for at least a 7 3/4.