GeorgiaBoy, Why didn't you just tell us what you have as Jon meaning is, the larger a tank one could setup is the better. As well you left out if you any attentions to house any tank mates with this fish for that too changes the tank size one needs to best suit its tank inhabitants.
As said as for the smallest I go is a 75 for one dragon eel with no main fish tank mates. For maybe you care to explain better in what you want to know for if you not a tank at this time for such a species, it might had proven better to ask what people think in tank size, or any other question or as well plans that you have for this.