Take her to an adult theatre. Get her used to porn in a room full of men. Go to a viewing booth and play with each other. This keeps it just you and her but others are "close by".
I would simply tell them, hey if you have happen to see mom and i in any of our home made porn and have any questions, you can come to me and ask. No weirdness, no judgement. Keep it open.
If your bf wouldn't be cool with the knowledge I would keep it to yourself
His dick size probably has little to do with unless you want to get with him again