If you are having these professionally replaced, the installing company will usually do the measurements for you. However, measure from the left window jamb to the right window jamb. Measure at the top, middle and bottom. Write down the smallest of these 3 measurements you take. That is your window width. Next measure from the top jamb to the bottom jamb. Do this along each side, add a middle measurement if you have wide windows. Again take the smaller of the measurements. The two smallest results is your windows size.