Sterlets get to about 4 foot in the wild, but 2-3 is more common. They are coldwater, don't do well in warm at all, probably the most common kind of death. In general they will keep eating up until 4-5 degrees, but probably best to stop just before that. During autumn before it gets very cold they are quite active trying to get as much food to fatten them up as possible, ready for the cold weather.
May I ask why you wouldn't put it in the pond now? They don't do very well in tanks, I've seen them get stuck in the corners, their "noses" get all bent and rubbed terrible and aren't tank fish. I'd try to do a bit more research first, they aren't easy fish to keep alive if you don't know their needs and it would be a bit of a waste of money for you.