I know you're looking at getting a toy as a way to prepare for intercourse, but I just wanted to mention a few things based on my own experiences.
By all means, get a toy and have some fun! But just because you're used to a dildo or vibrator of length X and circumference Y, doesn't necessarily mean that you'll feel comfortable with a penis of the same dimensions.
I enjoy penetrative intercourse and I enjoy insertable toys, but I have found through some trial and error that I require different dimensions and firmness in my toys than I do in my men for an enjoyable experience. The penis sizes and shapes that I find most enjoyable do not match the toy sizes and shapes that I like best and vice versa. A toy might have a realistic texture, look, and feel and yet still you might find your level of pleasure or comfort differs significantly when compared to intercourse.
I'm not trying to deter you. I just felt that this might be useful information to have. YMMV but, at least for myself, it doesn't seem like dildos would have prepared me for sex the way I might have thought. Although they would have likely helped to stretch the hymen and make things a bit easier in that respect. However, if you're just looking for a way to stretch your hymen a bit, you don't really need to worry about "average" size or girth, just get a toy you think you might enjoy!