The "whisper" range in NWN is 2.5 meters, while the "talk" range is 20 meters. If you want to eyeball it visually, a human model is roughly 2 meters tall in the game, so folks can hear a whisper a little over a body length away from you.
Incidentally, some measurements in NWN are given in feet and some in meters. The conversion in the game is 1 meter = 3.3 feet.