It is very hard to tell which Nep species is small and easy - because they all grow big with time and or vining. N. bellii is small but I am not sure about a beginner status. N. gracilis is a good candidate - it is small but from my experience it goes quickly into vining stage. Another idea is N. ampullaria. It forms a lot of basal rosettes. You can always cut the main stem when it grow to big and leave rosettes:)