stretching exercises are key, she will stretch to accommodate you! What you have to do, it go slow, one finger, two, three, four, different angles etc. Im small downstairs, but I have the ability to take quite a size. you just cant rush it. Slow stretching is key! and if she's really really horny, and you've done plenty of foreplay you'll be ok, it may well hurt, just be there for her, be responsive etc. because ultimately, she'll get physically used to the size.