N. campanulata. Perfect cute little lowlander. However, I have seen some mighty specimens of it as well...but is no where close to other neps. It also doesn't vine and forms offshoots on runners. Makes for a beautiful plant.
Try these small neps...
N. ampularia
N. argentii
N. bellii
N. campanulata
N. clipeata
N. glabrata
N. hirsuta
You might also consider N. northiana.
Even though it will get large, because it is so slow growing, you could keep it for awhile, and in that time your growing space situation could change so that you could accommodate it once it got larger.
campy is the only lowlander that i know that will fit in small spaces (and a mighty fine one at that!) my second choice would be belli which is also a LL.
many people say ampullaria is small, but it becomes a behemoth..