Here is a quick easy reference for the height, length, and fullness of any window treatment. (some of you may have seen this before, it is my tried and true reference for window measurements!)
The Height and Length of Window Treatment
Many designers prefer to hang curtains as high as possible - usually just below the crown molding - to make ceilings feel taller and elongate the profile of the window. But in some cases, with high ceilings or low windows, that can be too high.
Curtains should generally be floor-length unless they're casual curtains in a kitchen or café curtains. Most designers prefer curtains to just touch the floor, with a small, perhaps half-inch, break.
The Proper Fullness for Window Treatments
Fullness depends somewhat on the fabric and the style of pleats. The rule of thumb for most fabrics is that the finished curtains measure two-and-a-half times the width of the window, but for sheers and silk, designers often prefer three times for fullness.
Box pleats generally require fabric three times the length of the curtain rod for the correct fullness, while a flatter style of panel, like a scalloped top or grommet panel may need only one-and-a-half times the curtain rod length