I think things can be stretched a
little from absolute realism considering this is a porn game. I agree with your "8 inch average." Anything beyond 12 inches I would personally consider hyper, but if you were to stretch that baseline to 16 for the sake of gamey-ness and, again, the porn aspect, I think that would be fair. 16 is what the synth sheathe gives you, right? If we're being real, horse dicks are hyper. There's pretty much no way around that if you're using human capacity averages.
I think there's a serious misconception about how deep vaginas are, or how deep they can even physically be. There's organs up in there, you know!
I'm also well aware of the depressingly warped mindset towards dick size a lot of people have, and that bigger almost universally equals better (unless you're going to the other extreme with traps), but I personally would like male PC's or futas to be able to walk around with a 5.5 - 6.5 inch penis and not be considered "mini." Certainly don't consider them hung, either, but just like... everyone in the TiTSverse has foot-long dicks! I get the in-game explanations and rules allow for it really easily but it still bothers me. That shit's scary! I'm not using Pringles cans as dildos for a reason, why would I want something of similar size anywhere near one of my holes? Real sex has a size sweetspot, anything below is less stimulating but anything above is just downright painful!
But I'm a human, and my arguments really only apply to humans. With all the alien potential in TiTS there is totally room to allow for ridiculous endowments. They just need proper consideration regarding who they can and cannot fit into.