When I was around 5" girth at the glans and 5.25" girth at the end of the shaft I had a couple of girls with small mouths complain of jaw fatigue while giving me head.
I don't have a real big mouth myself, and I would have a very difficult time opening my jaws enough to allow passage of anything even 5.5" in circumference past my teeth for any length of time.
Obviously, peoples mouths differ in size with regards to jaw size, flexibility at the jaw hinge joint (temporomandibular joint), and tooth size. Some would probably have no trouble at all with 6 inches girth.
If a person's mouth is such that they cannot get a penis between their teeth, they can still lick it, suck at the tip of the glans, or place the glans inside their mouth in between their cheek and teeth and it still feels good. But for me nothing is better than the sensation of a woman wrapping her soft lips around my shaft and sliding them up and down, at least to just past the frenulum.
Never been deep-throated either.