You ask, ‘When is penis size too big & counterproductive?’
The simple answer is. When it fails to satisfy our partner.
I agree that petite women are less likely to enjoy units of 8 inches and above, but just because a woman is petite it, does not necessarily mean she will not draw some satisfaction from the pain that may be involved in over extending her. How we derive pleasure is largely dictated to us by our brain after all and not by size alone.
My first wife enjoyed/s deeper penetration. Even when my unit would rub up against her cervix - this was only an issue in specific positions, and she is a petite Oriental. Who when standing is about 5 foot 2” tall and who weighs in at around 100lb’s (or at least she did when I knew her).
My my most recent partner is a white girl and she is a full figured 5 foot 10” (UK dress size 12). This makes her at least three dress sizes larger than my first wife. The girl in question told me she liked big but thought “anything over seven was a waste”.
Go figure and a Happy New Year’s to all at Thunders!