It’s definitely going to vary by the hunt.
That said, a lot of people add more than a few inches to the height of jumps. I’ve seen a 20" split log become a 3’ oxer by the tea. On the other hand, if hunts are bold enough to claim 4’ jumps, I’d be ready for something vertical and north of 3’. It’s also important to keep in mind how much bigger jumps get when the approach is poor, the footing is ankle-deep mud, and you’ve been out for a couple of hours…
Ditto distance and speed. If someone is claiming either, they should be able to show you GPS logs.
All of that said, you and your horse need to be conditioning outside the ring. A lot, and in all sorts of footing. Otherwise, foxhunting will kick both of your butts.