Curtains and drapes tend to come in specific sizes, geared to cover standard sizes of windows. Before you do anything else, you should think about the size you'd wish for them to cover. Grab a measuring tape and find out the exact width of the window or other space that you'd like to cover with a curtain.
Before you're done, consider three additional mitigating factors:
If you're working with a header, you should find a curtain that matches the exact width of the header---so go with that number, not the window measurement.
If you're going for a full, bunched-up look, you'll want to get curtains that are longer than your windows or headers by about a third to a half. (This should only be done with thinner, lighter curtains.)
Finally, if you're not using a header, consider adding perhaps a foot to either side of your window to cover with the curtain. This will create an optical illusion that makes your windows look bigger! If you're taking advantage of this trick, add 24-36" to the width of your window measurement.
When you're shopping for curtains, shop with your measurements in mind. If you find that your measurement is between standard sizes, always size up!