I don’t think there is an answer to this. Every situation is different. Some girls love it, some hate it, some do it, some don’t. Within those parameters, it depends on other factors, how much you are paying, how long, does she like you or trust you enough, has she douched her arse, does she even care if it gets messy and you pull out with shit on your cock. I think cock size, whilst a factor, isn’t the only or most important factor.
I’m not a small guy in the trouser department, but I’ve had girls who say they don’t do it ask if I wanted to after seeing my hard cock because the want anal now. Equally I’ve had girls who say they will do it in pre meet comms, say I’m too large when we meet. BTW I’m not massive, but above average.
I only do anal when I want it, which isn’t that often, much prefer pussy, and if it’s offered. Meaning I very rarely book a girl specifically to fuck her arse. If the OP is looking for anal and nothing else will do, then he’s making a mistake if he thinks there’s some kind of average cock size where the likelihood is he’ll get brown hole access. He should just book one of the girls who offer anal as a dead cert.