I know that some of these answers have been posted in various locations, but it would probably be nice to have the answers somewhere central. I don’t know if the new book even answers these questions or not. Does it
Talk about base size? Or is it left up for interpretation?
Whole uncertainty: if you use original CD bull centaurs, you’d get regular cavalry bases on a rather thin model (like the centigors from the beastmen range). It’s hard to justify 3 wounds on such small models! BUT that’s the bases they came with. If this list makes them cost more and gives them more wounds, so be it. People cry about only two attacks instead of three, but they’re resilient and if you field them on their cavalry bases I see nothing wrong with having fewer enemy models that can attack them–in fact it’s usually a benefit for us.
Until FW actually release some models some of the base sizes are conjecture.
RAW say use the base size the miniature comes with which for bull centaurs would be cavalry bases. The new WF rules say that they are monstrous beasts and come in units of 3+ most people are not unreasonably predicting that they will come on a 40mm base and are using this, the same goes for the Fire-Born.
The destroyer has a note in the book saying the design intent is to be on a 100x150 base.