Use the graphics cursor or direct input to select a window -- the routine returns the equivalent width, defined as the integrated intensity in the window divided by the interpolated peak intensity.
Examples:
>> find-line-width<CR>
Type intervals, one at a time, CTRL(Z) to finish
Current units are km/s
# -4 27.0<CR>
Quadrant 1 Line equivalent width 3.86 km/s
..