I have a 4.5" one in a 90 and it seems like a great size for it right now. When it gets large i'm going to get rid of it or upgrade though. I dont think putting a niger over 7 inches in a 90 will cut it.
fishy is right I saw one that had to be at least 17in at my lfs and it looked crampt in their 240g tank. No way could it live in any thing less then a 180g. It was huge.