Hello Mojodex,
I am currently keeping a few species of eeltails at home so I will try to give you whatever advice I can.
As far as the size goes, as Jools says this comes down to the species - if it is T. tandanus then expect a good 18" or more in an aquarium. However other eeltails of the genus Neosilurus and Porochilus generally only get to about 20cm in captivity (although they are reported to 30cm to 40cm in the wild, this is uncommon), so in this respect they are fine in the average 'big tank'.
The two most commonly available eeltails (in Australia at least) are the Black tandan/eeltail Neosilurus ater and the Yellow fin tandan/eeltail Neosilurus hyrtlii. Both of these are great aquarium fish and you shouldn't have any trouble keeping them as they are undemanding with regards to water conditions - acid or alkaline is not so important to them, but they do prefer warmer temps of 26 - 29deg C.
Both of these species are gregarious when young, tending towards solitary when older, although they can still be kept in groups if given the space. They are not particularly agressive toward other fish, and when confronted the eeltails will be the first to flee.
Other species that you may occasionally come across are the toothless catfish Anodontiglanis dahli, which gets a little larger than the others; Porochilus rendhali, which is often found in the wild alongside N. hyrtlii and N. ater, but which is a shyer fish requiring dense vegetation (see
this thread in the French language forum); and N. pseudospinosus. There are other species kept by enthusiasts, but these rarely enter the trade.
I am currently away from home on holidays, but when I get back in a few weeks I hope to take a few pics of my eeltails and also write up a bit on them for the catelog, which currently has little on them. But if you have any questions in the meantime please do ask and I'll try to answer them.
btw any chance you can get a photo of the ones for sale at your LFS?
cheers,
Andrew