There is no single measurement that is used for all house windows, since it depends on a number of factors. The style of home changes the appropriate window size dramatically. However, common window heights are between 17 3/4 inches and 59 inches.
Common widths are between 19 1/4 inches and 69 3/4 inches. Most manufacturers produce windows in a variety of common sizes, including square and rectangular shaped windows. They choose standard measurements based on what is requested the most, though it is not uncommon for customers to order custom windows. If the window is only slightly off from the size needed, additional framing can be added instead of making a custom window.