Affecting a Room itself, rather than everything IN the Room?

Suppose I wanted to create an item that affected the Room it was in - say, an Imaginem item that made the room seem larger than it really was. Would I use "Room" as the Target, despite the fact that I am affecting the Room itself, rather than everything IN the room? Or would it be Individual, since I am really only affecting one thing (that is, the room) rather than a bunch of things within the space defined BY the room?

In this case you are actually affecting the species in the Room, so Room is a valid target. Room defines the space for the spell's effect, which is to make the room seem larger. This will require moving the species of the contents of the room as well, otherwise the illusion will not be very convincing (a large room with all the furnishings sitting in the middle and wall hangings sitting mid-air).

In other cases, Part will likely be more appropriate (transforming the walls of a room), as the walls of a room are part of a bigger structure. Note that the Structure target explicitly includes the structure itself, while Room target does not explicitly state this - probably because it is part of a bigger structure.

I argee with John. Imaginem is a bit less intuitive with this notion of species.
Regarding affecting specifically the wall only (with Terram spell for example), it should be Part with probably a +1 size modifier as Part affect up to Individual size volume of matter. So Part, because walls are usuall part of a greater construction (unless it is a one room, self standing house), and +1 because you are affecting more than 1 cubic pace, and very likely even a +2 depending how thick the walls are (thinking of medieval castle with very thick walls).
If you consider a room of 4m x4m (12 feet), and 2.5m high (8 feet), that's a total of 72 m2. Our modern houses have supporting walls of at least 20cm thickness, and medieval castle can have wall more in the 50cm+ range. So in volume, we are more in the range of 30 cubic paces/meters