I would use the same size exhaust on the vertical as the RF. It's easy enough to damper it. for the warming box / vertical smoker, A simple way to have it both ways would be to cut a hole and make a plug or cap to close it off. similar to the top of my fire box, I was debating doing the same.
this is a 6" opening, being directly under the chamber I think it would be more than enough. Hope I could help!