Well, generalizing African eels, really shouldn't be done, unless you are talking about the Ocellated Spiny Eel, aka African Spiny Eel. I say this becuase there are eels from Lake Tanganyika, notably Elipsifer that grow to be quite large. They'll need 100g at least when full grown. There are others from that area aswell, such as plagiostomus, and moorii which I am uncertain of their potential sizes.
TireTracks sometimes surpass the Fire Eel in size, and will therefor need a very large tank when matured. Both Tire Tracks and Fire Eels will need upwards of 100g.
Zig Zag Eels, if I remember correctly are actually one of the ones that stays relatively small in home aquaria -- usually never reaching over a foot. Don't hold me to this one, but I am pretty sure this is the case.