Can get around 20 cm in length and can be quite aggressive to other synodontis species. I have 3: a pair at 20 cm and 1 surviving baby at 8 cm. He is very aggressive to my two S. filamentosus, which are of similar size, but they get along as long as there is plenty of territory. A good catfish to mix with medium/large cichlids, as they can handle themselves.
A friend of mine had one of these guys for a long time. We used to love to sit back and watch him clean the fine sand, he'd take large mouthfulls of it and shoot it out of his gills, leaving two nice little trace lines behind him.
I have a featherfin in with my fully grown Oscar. They are the only two in the tank and they seem to get on fine. When I first introduced the catfish (the oscar had been in there for about a year on his own) they just went and hid at opposite ends of the tank, and nipped at each other a bit, but now they get on fine; I haven't seen them fight for months! They get on really well and when I am doing a water change the catfish will hide underneath the oscar while he lies down on the floor, which is funny to watch. A great catfish and pretty cheap too here in the UK (I bought mine for £10 fully grown). They can stand their ground against other large cichlids and will rarely be the victim of bullying.
I have come to the conclusion that most spescies of fish are like any other pet in that you can't predict their temperament. I am new to the hobby (3 months) but have researched and learned a LOT in that little time. Almost all the species I read about, including our Featherfinned Cats, when people post their personal experiences on the fishes' temperament, they range from one extreme to the other! I got my S. eupterus for my Mbuna tank. He was the first fish in there. Over the next two months I added six of the most peaceful Cichlid species. I eventually had to remove my poor Cat (larger than any of the Cichlids) because he was constantly picked on.
I just bought one of these little rascals and he polished off my snail problem (woohoo). Although he may and will be kind of aggressive towards smaller, less dominant fish, he is a blast to watch.
I had a large setup a few years back with various Synodontis. However the Featherfin is by far my favourite. I had a full size one about 18 cm long at least. He kept all of my other fish in check, and was actually very tame, and certainly knew when it was time to be fed. Remember to keep any Synodontis in a larger tank, as they like plenty of swimming space, and a good hiding spot.