From The Pilot's Handbook by Scott Todd

Chapter 5: Communications


When told, "Report a three-mile right base to runway 19," you would fly so as to wind up with a base leg that was three miles long, to a final that was the normal (1/4 to 1/2 mile) distance.


When told, "Report a right base to a three-mile final," then you would be setting up for a three-mile straight-in, at a normal (1/4 to 1/2 mile) base leg length.