

Nitrogen would be better. CO and CO2 both affect the human body in negative ways that tells the person “hey somethings wrong” or confuses them enough into thinking they’re fine.
The human body has no way of knowing if it takes in nitrogen though, so you can be perfectly fine conscious and happy one second, the air changes to nitrogen and suddenly you’re on the floor unconscious. If nobody collects you in time, well your brain can’t live without oxygen.
You can get a pretty good idea of someone’s size by their head and neck. Larger and taller people have bigger necks, and fatter people have… Well, more fat.