The Mountain Tamer Quad lets you get down to a 16-tooth granny gear, although in the fourth position.
For the upper range, it depends on the crankset - if you limit it to only rings in the third position I suspect a 74BCD 30-tooth would be the largest commonly available. If you include the inner ring of a double crankset they can be as large as chainrings are made - 60-tooth rings can be purchased and you can get plans to machine your own chainrings up to 92 teeth.