Most houses are built using only 3 to 5 different size windows, of the same make and model of window and window screen. Usually the kitchen, laundry room and bathrooms make up 1 or 2 different sizes. The final 2 to 3 sizes are made up of the bedrooms, living rooms, dens, and dining room.
This is important because by knowing this, you will not need to measure for every window screen in your house. If you know which windows are the same sizes, you can measure one of each different size, instead of measuring every window. Or if you do not have the current window screen of the window you need, but have the same size window screen on another window in your house, you can remove that screen, and follow instructions above to ensure proper fit, frame size, hardware choices, etc.