There are just a few things to consider when measuring for your curtain rods. I will break these in to separate sections listed below.
Space needed for the stack: First, you need to consider how much you would like your curtains to carry on past the window opening. This is important if you want to expose as much glass as possible. For example, lets say you have a window that is 72” wide and your curtains split in the middle. When your curtains are bunched up together on each end the "stack width" is 8 inches thick on either end. If you wanted this to be pushed all the way over so none of the glass is covered you would need to make sure the curtain rod is at least 16 inches larger than the original opening size of 72 inches. So your total curtain rod length will be 88 inches. It is not necessary for your curtain to always clear the glass and this is more often used when it is a sliding glass or patio door, or a window with a view.
Rod height: You will want to hang the drapery high enough so it covers the top of the window by at least a few inches if possible, if you are using tab tops, clip rings, or tie tops, which will lower your drapery down below the rod, move the rod up a bit to make up that difference. If you want a taller, more vertical or palatial look, you might want to hang the draperies a little higher, or even from floor to ceiling. If you are using ready made draperies, check to see what sizes are available, and decide on the rod height accordingly. Most ready made drapery panels are available in 84” length to cover the most common doors and windows which are around 80” high. It is not uncommon to use an 84” drapery panel on a 48” high window. This will give you a uniform look if you have patio doors or sliders in the same room using 84” drapery panels.