That's rather the wrong question to ask - even if anyone were to give the smallest true eel, it's unlikely that it enters the aquarium trade regularly, if at all.
However, the Dwarf Moray, Gymnothorax melatremus, might interest you. It grows to roughly a foot in length and does come around every now and again. Plus you can keep it in a relatively small tank. 50 gal absolute minimum, preferably bigger.