Curtain Width and Depth of Return
To determine curtain panel widths, measure the width of the curtain track or curtain rod and allow about 6 inches for overlapping in the center. Multiple this track or rod width number by 2 to allow for bunching, pleating and gathering, unless you're using box-pleated curtains, in which case, multiple by 3. Divide that number by the width of the panel you want to use (the common sizes are 40, 42, 44, 45, 48, and 54 inches, with 54 the standard), and round up to the next whole number, which will give you the number of panel widths required. If an odd number results, then one width can be cut in half and added to the outside of each curtain. Use this procedure if you have functional curtains and/or need to cover the entire window. With decorative draperies that don't close, you can be more flexible. Avoid skinny drapes, however, since wider panels will provide the illusion that the room is longer and more balanced.
The "return" is the distance a traverse drapery (one with draws and pulleys) must return to the wall to cover the gap at the end of each drapery created by the outward projection of the brackets. The standard return is 3 1/2 inches, but with window treatments over 9 feet tall, you might need a deeper return, as much as 8 to 10 inches, for the drapery to hang properly. Using a deeper return places the window treatment out from the wall and allows deep pleats and folds to enhance the richness of the fabric.
Pleats and Fabrics
In rooms with high ceilings, use richer, bolder colors for window treatments and luxury fabrics, such as silk, taffeta or velvet. If a pattern is desired, choose a horizontal or railroad stripe that will appear to give the curtains more volume. If you use pleated fabric for window treatments, the pleats should be over-sized, at least 6-inches wide. Pleats that are too small may be hard to see. In large rooms with wooden floors, consider adding sound-absorbing interlining to draperies to avoid an "echo chamber" effect.
Valances and Other Window Toppers
For window valances, cornices and other window top treatments, use the "Rule of Fifths or Sixths." Divide the finished length of your drapery by five or six to determine the finished length of a valance or swag. The valance should cover the header (pleats or shirring) of the drapery, as well as the hardware and frame of the window. If you have double-story windows, consider a valance or curtain rod above the lowest tier of windows, which will create a horizontal line around the room and bring the scale of the room down.
Rods and Tiebacks
With oversized windows, drapery rods and rings must be oversized, too. For instance, a twisted length of wrought iron rod that would look good in a standard-height room won't appear substantial enough in a room with vaulted ceilings and large windows. Use curtain rods that are at least 3 inches in diameter. If the windows are tall but narrow, adding an extra-wide curtain rod will give the illusion of width.
For tiebacks, use the "Golden Mean" ratio. Either place them high, about a third of the way down the drapery, or place them low, about two-thirds down. Tiebacks in the middle are less attractive because they cut the fall of fabric in half.
Choose drapery panels that reach from the floor almost to the ceiling, which will soften the room and also absorb sound. At the very least, hang your draperies as high as possible to make the room look more balanced. Leave several inches between curtains and ceiling, so it appears you're dressing the window, not the wall. If you can't afford functional window treatments--ones that close--make them decorative instead. For example, use inexpensive cotton fabric with a tassled edge to create side panels that always are open. Since most ready made draperies average 72 to 84 inches in length and only a few carry 96 or 108 inch draperies, if your windows are longer, you may have to consider custom or home-made treatments. Some high-end stores like http://www.dreamdrapes.com have draperies up to 130 inches long.