If you don't have a prtruding window sill the curtain drop ideally would go 6-8" below the sill.
(not my work. Found this on pinterest ;))
Curtain panel can have several different functions. The function you choose will determine its necessary width. They can be either non functional, also known as side framing, or they can be functional. Side framing curtain panels usually serve to frame a window or accent an exsisting treatment such as an upholstered cornice board or even a roman shade. The width of this style will depend on how casual or how formal you want the room to feel. The narrower the curtain the more casual feel you will achieve. The more fullness and width the more formal and dramatic feel you will achieve.
For side framing curtains you can go as little as 1/2- 1 times width. One times width is typically 50".
The window in this picture is only 38" wide. So we went with half widths, each measuring 25" wide, creating a casual look. (curtains were for a client's window and not meant to puddle*).
For curtains that are intended to function, there is a general rule to consider when deciding on width. A curtain of one width will generally cover 25". So one set of curtains single width will cover a window with a max width of 50". If your window is wider than 50" it would be best to add a half width or full width to the panel for more fullness.
More fullness will ensure that you won't have to pull the curtain completely flat to cover the window. In addition to a more formal feel, more fullness will also ensure that your curtains don't fade from the sunlight.
The final thing to consider with curtain length is that ideally you want to hang your curtain rod between 5" and 8" above your window's frame.
All of these factors should be taken into consideration when deciding what width and what length you will want your custom curtain to be.
Happy window dressing!