Years ago, most drapery panels were pinch pleated and were used on traverse rods that opened and closed them. They came in lots of widths. Gradually drapery panels started being used as stationary panels to dress the sides of windows. The width became less important because the panels just sit there rather than being functional.

That is a very short explanation for what is a far more complicated subject.

Will you be closing the drapes? If so, the rule of thumb is to buy double the width of the window. So if your window is 36" then you'd want two 40" panels. If however the drapes will remain stationery and open, just buy one panel for each side.

Or if they are in a bedroom and you will only have them closed when sleeping, then save the extra money and use the standard width. They will be fairly flat (not many gathers) when pulled closed, but will cover the windows blocking the light.

But Annie is correct. If you were to have made to measure drapes, they would be between 1.5 and 2.5 as wide as the window opening, depending on the fabric weight and style of the treatment. So your 72 inch window would have panels on each SIDE that were 90 inches wide. Here is the issue. Fabric only is woven in particular widths. So going wider than the standard width (which is generally the width of a single panel in ready to hang drapes) means you have double or triple the fabric expense, and also additional labor to sew the fabric together to make a larger panel.

Are you looking for sheers? Draperies? Did you want to close them? Other sizes are available lots of places. I've shopped here but not for a really long time. They have coupons all the time. Sign up for their email.

Here is a link that might be useful: Linens n things

Curtains are to be 2-3 times the window width. Draperies are different. Seek the help of a professional, whether you buy from them or not. The head of the drape determines the size plus (I believe) 10 inches. Seek help at halfpricedrapes.com. They are a great source.

If you're looking for a budget solution, you can double up the panels on the wider window. If you are not going to open and close them, you won't notice that there are two pieces on each side. If you want to be able to close them, you can sew the panels together.
Ready made packaged panels come in limited widths and lengths and it's difficult to find them for wide or tall windows unless you go to some of the online catalog stores. Some will carry a larger selection of styles and sizes.
Custom curtains and drapes are expensive.
Good luck.

The general rule of thumb is to use two times your window width of fabric. But if the panels are going to be kept open maybe one will be enough for your application. If you are not going to open and close the curtains all the time i would also sugest a nice set of hold back or tie-backs. This will make the window pop and make the room look bigger. I just got some sheer curtains from Swags Galore and the price was great considering i was on a budget. Hope this helps!