Curtains obviously cover windows and there are two types of window treatments that cover the windows. One type is fabric from side to side, the other option is a fabric or blind from top to bottom. So, you need to first decide what type of window covering you wish to have. Some people even layer their window treatments and have blinds and curtain panels.
When ordering blinds, go to a blind web site and they will give you instructions on how to measure your window to get the proper fit. You need not order from them, but at least you will learn how to order the right size.
For drapery panels that go on a rod, you will typically need two drapery panels that slide from side to side to obtain your privacy. Most drapery panels come in typical sizes to fit most window widths and lengths. You need to decide if you want your drapery panels to go to the bottom of the window sill or any length between the window sill and the floor. If you go for full length drapery panels, then you will typically need 84" or 96" panels, depending on the height of your windows and your ceiling.
I like to hang my drapery panels on the wall, above the window as this gives the illusion of height, which is decoratively appealing. if you would like to maintain more glass for sunlight purposes when the drapes are open, then hang the rod slightly wider than the window to allow the draperies to stack on the side(s) without blocking the glass.
To get the right width in your drapery panels so they look good on a window, you take the width of the window and times it by 2 to 3, depending on how full you wish to have your draperies when closed. Some drapery panels that are hung with blinds, need not close as the blind is used for privacy, so the drapery panels remain stationery.
I suggest you go to Overstock.com or other on line resource to check out curtains/blinds to see the different styles and prices.
If you have the budget to have your curtains custom made, that is a great option to get exactly what you want. Most custom made single drapery panel will cost about $50. each, not including the fabric and lining (if you want them lined). However, you need not go the custom made route because there are literally thousands of drapery panels available in all sizes and fabrics.