Don't bother.
My cock is only 5.5" long and out of all the women I've fucked (roughly 50) I've only had a single one say she wishes it was bigger. Porn-star level penis size is as overrated as freakishly huge fake tits.
FWIW, the one that said so was extremely loose-- I could fist her with no lube, just her own wetness. I've been with a few that were somewhat loose and they had no problem with me, it was just the worn out one.
6" is fine. Why risk permanently damaging your dick to please a loose whore when any normal woman is happy with it?