Girth is a strange one. My gf says her fingers couldn't reach around her ex's cock & that she estimated a good 6 inch circumference (it's what he claimed as well). But 20 years later, with a noticeably looser vagina, she simply cannot take a 6.25 inch dildo, no matter how soft or lifelike. She can barely manage 6in girth dildo before telling me to get it out due to pain & only starts to get comfortable at about 5.5in. She says by the end of her relationship with her ex she was taking his 6 inch circumference with ease. So I don't know - I think sex with a real penis and a dildo of 6 inch circumference is always going to be different, with real cock being easier to take regardless of dildo quality.
As to the question, based on what my gf & my exes have told me (not that I've asked all of them but I did speak to a few who were open to it), 6 circumference is very uncommon. My one ex who had a very adventurous sex life & slept with lot of guys, says only one was so thick that get fingers didn't meet, suggesting 6 or thicker. Of course, hand size comes into this too, but it gives an (admittedly very unscientific) idea of how rare 6 inch girth is (which shoukd make us thin guys a but grateful given the female preference for thick girth).