Position a metal tape measure in the upper end of the window opening. Place the edge of the tape measure against the inside edge of the left window jamb, and extend the tape measure horizontally across the window opening to the right window jamb. Measure the width of the opening to the nearest 1/16 inch. Measure in this same spot again, to verify your measurement.
Place the tape measure against the left jamb in the middle of the window opening. Measure the window opening from the left to the right jamb. Measure again to verify your measurement. Repeat this same process at a spot near the bottom of the window opening. Measure twice to verify your measurement.
Take the smallest out of these three measurements, and divide that number by two, which gives you the width for each shutter. For example, if these measurements are 31 inches, 31 1/8 inches and 31 3/16 inches, divide 31 by 2, and use shutters that are 15 1/2 inches wide. This gives the shutters the appearance of being operable, even if they are only decorative.
Measure the width of each window separately, even if the windows appear to be the same.