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Are there any knifefish that are suitable for a 29 gallon tank? African brown knife, etc? Or do they all get rather too large a size?

Correct me if I'm wrong, but both black, and brown knifes get quite lage. Couldn't telly ou how large, but the biggest I've seen is around 4-5 inches I beleive. If you want a very impressive large knife, clown knives are very pretty but grow very large, again, I' not sure on how large, but they should be pkay in a 29 gal tank.

actually the brown knifefish, X. nigri is a fairly small knife. reaches a length of about 8 or 9 inche, it's the smallest species I know of. http://fishbase.org/Summary/SpeciesSumm ... name=nigri

I have had a black ghost knife fish in a 29-gallon tank for about a year now. When I got it, it was about 4" and it doesn't seem to have grown at all, but it eats well, looks beautiful, and is very active at night. I have seen various reports on their maximum size--anywhere from 9" to 22." My LFS told me that the fish would grow very slowly, though, so I could get it if I had some plans to move it to a bigger tank in the future. Like I said, I've had it for a year, and it hasn't gotten any bigger. It's a great fish.

If you like a single, easy-to-keep Knife; Xenomystus nigri will do fine.

You can add just one to your tank, but only if you don't have Neon-sized fish....it likes a snack now and then.
apart from that, pretty docile, strong and nice fish; I had a few, for years.
They can reach 8" or so, but it takes them a long time!

Other smaller Knife's include Sternarchella and Steatogenys, but these are very rare and not easy to keep. They look like little brown versions of Ghostfish (Apteronotus) or Green Knife (Eigenmannia spp).
Apart from the Ghost (wich can reach anything from 1 foot to 45 cm...), most South American Gymnotiformes (Electrical Knifefishes) are not easy to keep for a longer period.

I'd stick to Xenomystus if I were you.