I think sturgeons are a game fish in some areas. If I'm not mistaken they get HUGE....several feet in length. They look almost "pre-historic". I think they get cavier (sp) from some species of these fish. I don't think they are a fair candidate for the typical home aquarium or pond.
Seen 3 or 4 in a LFS tank once. They were about 5-6 inches long and were going for $50-60. Not worth it IMO, they will outgrow just about any tank very quickly. They get like 7-8 feet at least if I remember it correctly.
All over the place, depending on the species. Do a Fishbase.org search using "sturgeon" as the common name...I was surprised at the number of varieties.
There was a thread about Sturgeons on the old AC...If i'm not mistaken there are some smaller varieties out there..Not sure if they are made available for commercial sale....Bear in mind that a smaller variety may still be 3 or 4 feet...Big sturgeon can hit 14 feet...They are very interesting though. Here is a beauty...Not sure if he'd be suitable for the average hobbiest though!