Most woman I have fucked can accommodate 8 inches with no problem, beyond that it depends on the desire of the woman and the skill if the bull wielding the cock. If you are experiencing your first big cock I wouldn't recommend anything much bigger than about 8 thick inches. If that proves to be fun and you enjoy a hard fucking from that then move up. Women generally have a lot more capacity for cock than they realize but it doesn't hurt to move up gradually. A large dildo may help you find out your cock capacity, but no dildo however well made will ever feel as good a a real cock filling you