This isn't a good toy to choose for insertive play, as it could slip out of your hands, up inside, and make for a potentially anxiety-filled occurrence.
Sizes is a very personal thing. There is only one hard-and-fast rule: you don't want to get something too big. If it's too big then it won't be fun, and you run the risk of intentionally or accidentally "forcing it" and causing problems. Play around with fingers and see how big you are comfortable with, then start from there.