Eh… Well clearly there is no single answer. I expect it to be a combination of physical conditioning and mental comfort. My girth was always 5.2”, I asked my girl for anal several times and she only did that once before, which wasn’t a pleasant experience according to her. She was still open to it though, and I fingered her in the butt from time to time. One day we went to a hotel for a weekend away, took a nap, lubed up and I eased in there real slow. The third time we did anal we were high (just weed). She’s been asking for more anal ever since. So yeah I guess mix some ass play into your regular sex life without it being actual penetration. When she’s used to her ass being played with, suggest penetration, use lube and more lube and make it nice and comfortable. And of course she gets to call the shots. Don’t just ‘get up in there’ like a neanderthal.
Just my personal experience on the two main questions here: (1) what’s too big (2) How do you get your girl interested.
February '16: 173 mm BPEL (6.81") 132 mm MEG (5.20")
November '18: 190 mm BPEL (7.48") 137 mm MEG (5.39")
Goal: A sustained 7.7" by 5.8" during intercourse