I had a clown tang in my reef for about 6 months or so but I got rid of him because I didn't really know how big he got until I had already had him. A friend of mine had a much bigger tank and had been wanting one. I traded mine with him because he had bad luck with them in the past.
They are very aggressive. I would say they could even be more aggressive then a Sohal tang. I added one that was about 4 inches to my tank that already had an established Sailfin tang. The minute I added him to the tank the CLown tang let the sailfin know that he was not going to be pushed around.
CLown tangs get pretty big, I think they can get around 38-40 cm, which is in the neighborhood of 15-16 inches or so. Having worked at LFS in the past and being active in the hobby, I will tell you that not many people have great success with this species. They are delicate just like the achilles tangs are.
But people do keep them and have some success. It is a mixture of having the right tank for them, picking the right fish and luck. I would say, that you need a lot of swimming room, at least 8 feet or so. They are really skittish and will dash all over the tank and if it isn't big enough they will run into things and hurt themselves. You also need serious circulation and aeration. They like movement.
Also, selection is imperative. I mean, a lot of these fish come into the trade and not a whole lot of them make it. I aggree, I don't think I would get one smaller than 4 inches. I don't think I would buy one that was full size though, not that you are likely to find one that size anyways. I would look for the usual things like color, thinness and feeding. I would be very, very carefull in selecting these specimens. Follow your gut instincts. If you have any sort of bad feeling about the fish in question, don't buy it.