I’m assuming the question is for any gender (not just fellas), so my own preference has always been Barbie size.
Around eleven inches works best for my own imagination, give or take an inch or two if I’ve lost my wooden ruler.
Main reasons being, is I like creating handheld imagery or general interaction with large hands. So a woman of that size can easily be held in one hand, but there’s enough of her still on view to admire and appreciate… etc. wink