I searched "rooms z-levels" and "rooms floors" (or something) and didn't find anything. If I'm still suggesting something that has been brought up a thousand times before, I have a bucket of elf beef to sacrifice to Footkerchief for repentance.
Anyways, currently in DF you can only designate rooms in two dimensions. You can't, for example, make two rooms on different z-levels, connect them by a ramp or staircase, and designate them both as a single barracks.
Or to use my fortress as an example, I have several rooms that span multiple z-levels. One is a cathedral with two floors (made by channelling out the z-level above the "cathedral level"). Another is an "inverted pyramid" design that goes down some z-levels, each z-level two tiles shorter in length and width than the one above it. I'm connecting the levels with staircases, and I'm going to line the edges of each level with coffins.
In both of these cases, I should be able to designate the entire room, across z-levels, as a single room, or let the game do so for me automatically (though care would have to be taken here to avoid exploits). Measuring a room's size in cubic tiles instead of square tiles would reward you for making huge cathedrals, pyramids and the like, and could generate happy thoughts to the nature of "Urist McWorker was amazed by a truly cavernous dining hall recently". I don't know if dwarves think like humans do when it comes to these things, but at least in the LOTR movie, Moria has plenty of vertical space in the chambers you're shown.