I think that you should had called the topic to your question, "Eel tank Requirements" rather then eel size.
Its like Andy says about inch per gal and these requirements raises some depending on the species for many or a good number of folks will tell you that large angelfish requires a huge tank over many species. That is as long as were not talking sharks or so.
Many you will see keeping a sfe in as small a tank as a 55, I in noway can ever agree with something as this for my bare minimum be a 75 gal tank and that the larger one could go with would be better.
And in keeping other large marine species, the tank size needs to become much larger in gals, but that is my thoughts on the subject

lol