Mosley,
Think of it like this; In the NORMAL mode, when you press the pinpoint button and swing the detector over the target you have found, the sensitivity of the detector will decreases a little bit each time you swing the coil over the top of the metal object, until you get just barely a whisper after several swings which helps many people 'pinpoint' the target better.
In the SIZING mode, the detector will NOT decrease in sensitivity as you swing the coil of the target. This allows you to get an idea if you are over a large target, small target, a long and narrow target, and therefore size and shape the target using the pinpoint mode. You can watch the Intensity meter that comes up near the top of the screen as you do this. The pinpointing segments come in from the left and right as the coil becomes centered over the target even if the audio level seems like it is at max.
If you are over something big, you can 'detune' the pinpoint mode by starting the coil out partially over the target when you press the button and begin pinpointing. This effectively starts you off with less sensitity. Conversely, if you are having a hard time pinpointing something and can't seem to find it, you may have unknowingly started your pinpointing directly over the target, or something large nearby. Try moving the coil to a clean area and repressing the 'pinpoint' button, and begin your pinpointing again.
It was suggested above that you try both modes with a piece of pipe or similar and you will very quickly see what each mode will do.
Let us know how if that works for you.
Rich