It is simple to choose the right curtain size; it just requires a few quick measurements and a determination of how you want your new curtains to look. You will need to consider both length and width when you are choosing the curtains; first, measure the width of the window. The best way to do this is to simply measure an installed curtain rod, since that will actually be the width that the curtains are covering. Then measure the length of the window, and decide if you want the curtains to rest at the sill, below the sill, or even near the floor.
When choosing curtain size, measuring the width of the window or of the curtain rod is easy; most people find it simplest to use an aluminum measuring tape for both shorter or longer distances. Once you have your window measurement, you will want to double it, and this is the width of the curtain or curtains that you are looking for. To use a simple example, if you've measured the width of your window to be 20 inches (50 cm), you will want to choose a curtain that is 40 inches (101 cm) wide; or two 20-inch curtains that, when placed together, create the desired amount of width. This does not have to be exact, but it is a good idea to at least double the width of the window to find curtain size. If you choose curtains that are the exact same size as the window, they will tend to look stretched out and unattractive, and will not cover the sides of the window.
The length of the curtain size is more related to personal preference. More formal rooms may feature curtains that reach past the windowsill, almost to the floor, whereas a more casual room might feature curtains that only extend slightly past the sill. Typically only when the curtains will be sitting inside the window frame, such as for cafe-style curtains in a kitchen or bathroom, will they rest a centimeter or two above the windowsill. Otherwise they will look too short.
Keep in mind when choosing curtain size and selecting a pack of curtains that the pack will specify the measurements for each individual curtain, not the pair together. Be sure to measure carefully when choosing curtains, and keep the curtain fabric in mind as well. For instance, with sheer curtains you might want to choose ones with additional width to make them appear more substantial, whereas heavier curtains may not need as much.