It is more about, that I would like to see, that the size has at least some meaning other than a few words in the description and a difference in the combat relevant stats would be interesting as well but could make the fight unnecessary hard.
That I written about the sex as the place to implement the use of the size difference is just because, that right now I could use a Centaur to breed a Pixie (with the Breeder class) and everything is fine, and as there is nothing that really tells an exact size, to me it feels like the Pixie is at best half the size of the Centaurs penis. Meaning that the furry-porn-logic would apply here, where a person can be impaled by something wider and larger than themself without any harm.
Also, the moment I had started to write the example, the post had become more text than intended.
I had written this last part and (as I see now) I deleted it by accident. 😅