8"x6" I've learned any thicker will be too much for most women. I got my wife a 9" dildo and it was too much(thick) when i measured it i was surprised it was 5.5"(1 inch more than me) that 1" sure makes it look a lot thicker. Not sure if it was the thickness or that its so hard and wont give in a bit like the real thing.
I am 8x6 and besides girth one inch longer wouldn't make it harder...