The first choice you’ll make is between blinds and curtains. If you choose curtains, go with a length that lets you hang your curtain rod several inches (at least) above your window frame; this makes your windows look taller. If you want your curtains to touch the floor, choose a length that gives you 1-3 inches extra for pooling at the bottom. When deciding on width, determine whether you want the curtains to fully block the window when they’re closed. If so, each panel should be as wide as the window so you end up with twice as much fabric as glass. If you aren’t planning to close the curtains, then each panel should be about ¾ as wide as the windows, so that your end result looks fluffy but not overdone.
If you’ve decided to go with blinds, then your measurements will be more straightforward – you just need to choose whether they’ll be mounted inside the frame or outside to determine the width. If you’re having blinds custom cut, then the dealer will help you choose the right size; if you’re buying pre-packaged blinds, then you can often pick your width and then customize your length at home.