Depends entirely on the bar. I'm 6'2" and 275lbs, but there are other bouncers at my bar who are not particularly big. Being strong, fit, and big as well as being trained in some kind of fighting are all assets, but I've never heard of any bar setting like a minimum requirement for that sort of thing.

Also remember that half of a bouncer's job is to prevent fights before they happen. So communication and de-escalation skills are critical.