This truly depends on the personal preferences of both partners. While the vagina can stretch to accommodate almost any size penis, that does not mean it will be comfortable. The length of the vaginal canal can vary, and many women find a penis length that hits the cervix to be very uncomfortable. Girth, on the other hand, is a different story. Time and again, studies show that given an option, women prefer a penis that is of average length but of larger girth!
See more about penis girth enlargement here.