Large enough where it's not like juggling two basketballs or pulling on the teats of two regular shopping bags crammed with as much Mozzarella cheese as you would fit in them, a size that's practical enough at most to provide me with Cattleya mommy milkies and not be an obvious major hinderance to her.
I think that anything akin to the ill-fated Lolo Ferrari of Eurotrash fame where the look is cartoonish is where I draw the line, she might have been a nice lady but for me every aspect of her sexually is such a massive turn off.