The existence of G spot as a distinct structure is still doubtful but only the outer one third of vagina including area of G spot is having maximum sensation than inner two thirds of vagina.
The clitoris and around is the area of maximum sensation and to stimulate that the length of penis is not important.
Though the vagina is distensible during stimulation it can hold any size of penis.
If the size of penis is more than 2inches there shouldn't be any problem in penetrative sex and mutual satisfaction. I hope you are convinced.best wishes