Small Nepenthes? I've been doing similar research myself
In my opinion, there are 2 kinds of "small":
1) plants that don't vine much and stay compact
2) plants that do vine (have long internodes in the vining stage), but because their leaves are relatively small, they still occupy little volume and could in theory be allowed to snake around a terrarium without overwhelming it. Contrast that to a mature Miranda which, no matter short you prune it, will always occupy a lot of space because of the leaf & pitcher size in the rosette stage.
So far these are the taxa that I've found that, judging from the pictures I've seen online, could potentially do well in a smaller grow area:
Species:
campanulata: compact, doesn't vine, and still has small diameter
argentii: does vine eventually, but it's a small plant
adnata: vines, but it has small dimensions overall
talangensis
aristolochioides
vieillardi: the clone out now does not seem to get very big
Hybrids:
thorelii x alata Kondo: A dwarf. I grow it, and I expect it to take several years before it needs a pruning.
talangensis x veitchii
muluensis x lowii: slow grower, and takes a while to get any size
talangensis x tobaica (and the reverse, called Pisacho): vines, but doesn't seem to occupy a lot of volume
ventricosa x talangensis
ventricosa x (dubia x singalana)
tobaica x thorelii (and the reverse): gets viney
Mae West: hard to find......
Kristin Ammar
Those are the smallest ones I have found so far. You might think that thorelii x aristolochiodes would be small, but most seem to say that it is a vigorous viner, so I'm not sure. Aristolochioides does seem to have a "shrinking" effect, however, on most of its hybrids.
I haven't grown most of these plants (yet) -- these are just my observations after doing extensive searches for images on various forums and photo sites. Hopefully that helps you in your quest