1) It is a radius, centered on the trigger/trackmark location (TS12+SP1 added a very handy semi-transparent shape - red for trackmarks, green for triggers - showing its extent along the track);
2) The radius is expressed in meters; the default value is 20 meters = 65.6 feet.
 
3) You can set the radius down to 0.01 meters (3/8"), but don't set it at 0 (the game will not recognize it).