After about five years of being a tgirl fan I've come to the conclusion that the best size is around 6'' or 7'' max...
I've taken a few large cocks in my ass (large meaning around 20-22cm or 9'') and although I liked it I would never do it again.Let me explain:
What we men like when we get penetrated is the strecth in our asshole and the rubbing of a cock against our prostate.A ''normal'' sized cock can do that pretty well even if it's not a monster like Mariana's.
The girth plays a big role in how "full" you feel when you get penetrated,length can actually hurt rather than feel sensual.I recently took a 9'' monster cock in me and I had to wait for about 10 days before I could feel ok again,I couldn't sit anywhere without a slight pain and "open" feeling.
I get better satisfaction with smaller cocks simply because the girl can fuck me hard but not painfully.Just my thoughts.