also depends on the shape of the head as well. Some people have an egg head and a large fits weird. I have a pumpkin head so it sits and fits way better.
I have an egg head with 22.5 inches above the brow for the FG5000 - fits perfect and there's about 2 inches left of space at the back where the laces help. The headgear sits tight on my forehead, rests on my cheek and hovers slightly infront of my nose - it feels perfect.