The first thing to consider:
You should first think to yourself what you want the function of your treatments to be. Do you want them exclusively for decorating purposes? If so consider a treatment such as a valance, swag or other treatment that frames the window frame as opposed to covering the windo itself.
Do you want them to insulate or block light? If so, you will want to go with a curtain or shade that will cover the entire window. In this case, windows should be easily accessible.
Do you open your windows often? If so, you will need to pick a style that allows easy accessibility to the window itself. Also, you will need to consider the various mounting options of a window treatment. For example, inside mount roman shade may prevent a window from opeing or closing all the way.
The next thing you should consider when choosing a style is
Decorating your home is an expression of yourself, your personality. Choose a form that reflects your personality. You may want to choose a curtain with a dramatic puddling at the bottom, or still and nonfunctioning curtains with a high profile decorative curtain rod. With custom window treatments you can express yourself and your personality in all the decor in your home.
Go here to curtain length recommendations. (Post link coming Friday).