Okay, so I've been mapping for a bit now and there are still some things I'm not entirely clear on, so I figured starting a thread so that everyone can see the answers was better than inquiring privately.
Anyway, the questions I have (so far, at least), are:-
1. How much room do boss rooms currently need to have (height-wise in intended "boss corridors", at least for ground bosses)? I'm currently thinking 9-tile high is a good minimum, but that's more of a guess than anything. (Additionally, what's the minimum height for overlords, since I believe it'll be different to normal bosses?)
2. What is the difference between Low Atriums and Large Atriums? Do they get used in different places, or is there some sort of dividing design principle between the two?
3. Is there a difference between Stairs Up and Stairs Down? (I suspect it's whether more door spawn points are on the higher areas of the room (for up) or on the lower areas of the room (for down).