While the question is tired, you posed it with numbers and it makes me offering an answer pretty simple by comparison.
I’d modify your girth chart. Basically start where you are but do the increments quarterly.
Girth:
5.5”-5.75” above avg or “thick” (where lube starts to be required)
5.75”-6.0” Big (where you really ought have lube)
6.0”-6.25.” Huge (where if you don't have lube it actually starts to friction hard enough to be uncomfortable on the shaft)
6.25”+ Monster (complications that need to be talked about, more often than not)
I modified the chart because…I’ve been through these numbers. Most women start having trouble, spotting red at around 5.7” on the sheets. Verified by Seneca32, who also has similar issues at that size range.
I’ve seen blood en coitus at just above that. Then seen a fair amount of blood into the low 6” range. All with women who insisted on no lube at first. My lower 1/3 is 6.5-7.25” or so in girth and so the base doesn’t fit in most women. As in session has to get really long and involved for them to open up wide enough. And either it’s uncomfortable or still won’t go, due to bottoming out.
The typical vaginal opening is around the side of a dime. Smaller than most fingertips and still snug. You propose a doorknob (at 6.25” or so) as just “Big” and…let’s just reiterate, most objects women pick up and think are “thick” are under 6”. The Maglight D-Cell flashlight was the common standard for pretty much huge and it was around 5.6 or 5.75” (can’t remember, I measured it) and snug in the one or two women I know who tried it.
Lastly but not least…go to a toy website. Reviews will place your toys above 6” girth as “too big to use comfortably” more often than not and the selection drastically drops off above the 8” length and 6” girth unless you’re on a specialty site like Bad Dragon. Which markets to anyone into extreme insertions.
**Edit, P.S.
PEforFun, who pioneered the full vacuum extreme pumping idea here on Thunder’s, mentions that he wishes he had stayed around 6.5” girth (instead of his ending size at around 7.2” I believe) as that was the largest he recalls being that still was able to penetrate his partners. Which would make 6.5” the absolute limit of even feasible sex with most women. So that would be beyond “monster” and pretty much impractical for sex. So, it’s completely out of the question to consider 7” on the chart unless being “too big” is on your To-Do List. Being “too big” is a fine goal, if that’s your thing. But keep the perspective and make sure your goals make sense for what you actually want to do with your penis.