Great question JD…
Every PIR (passive infrared) motion detector has different sensitivities – both range and angle.
Most PIRs work by using a multi-fresnel (bumpy?) lens that creates a “moving shadow” effect on the actual sensor target. Movement across the “visible” field of the sensor fades in and out of focus and that affects the brightness of infrared light on the sensor, thus telling it there is motion.
It’s too bad that these don’t have a “reverse” mode that would project a visible light out from the sensor area through the lens so that you could see a pattern projected on the walls, ceiling, and floor which would approximate the width-scope of the sensing area. But you can possibly guess the area by looking at the lens and picturing what that light pattern would look like. Note: PIR’s don’t project even infrared light or sound, they just passively “see” the heat from people or animals.
It would also be nice if PIRs had interchangeable or focusable lenses, but they would be more expensive. The lens is just a piece of injection molded plastic.
I think that any sensitivity adjustment parameters (dials or configuration options) relate to how much “brightness” that the infrared hitting the sensor must be in order to trigger, which affects the distance from the sensor, but not its width.
Given all that, I’ve not even answered your question 
…
I think you just have to experiment with placement “map out” the trigger area by testing it while moving throughout the room.
Once you find the edge of the detection area, you know you can turn the sensor away from that area and that will move the edge in the same direction (i.e., picture moving a spotlight, movie projector, or camera viewfinder).
Your idea of putting the PIR inside a box is a good one: You can put a slit or hole in the box in front of the PIR lens, and that could help tune the focal width (the detection width) and let you more precisely “aim” the detection area. Also try different models / brands of PIR, as they have different lenses and distance sensitivities.
I hope I’ve been of some help!
…CP / Terry.