When measuring for your new drapery rods you should consider your window size as well as your drapery style.
Figuring out what size drapery rod you're going to need for your window treatment is sometimes a little more complicated than just measuring the width of your window. When choosing the size for your drapery rod, you'll need to take into consideration what style treatment you're going to hang as well as any obstacles near your window that might impact your installation.
To begin, measure the width of your window and decide if you're going to hang your drapery rod above the window, on the window trim or inside the window frame. If you're installing your drapery rod above the window, you'll probably want your drapery rod to extend past the window and trim by an inch or two. Make sure to include the overhang when selecting your drapery rods width.
If you're installing on the window trim, you'll probably want to add a few inches to your width as well. But, for installations inside the window frame, you'll want your drapery rod to be the exact side-to-size width. And, you'll need to be certain that the drapery rod you're interested in has matching inside mount sockets available.
Some drapery styles like swags may require additional drapery rod length past the mounting brackets. You'll need to make sure that your fabric, when gathered, can pass behind the drapery rod to the left and right of the bracket. If you choose a rod that's too short, your fabric won't stay where you need it be. Other drapery styles, like draperies with grommets, also do better if you allow several inches of pole past your mounting brackets. The mounting brackets can be used as an end stop for the grommets to keep your curtains in place when closed.
If you're using finials or end caps on your drapery rod, don't include the length of the finial when selecting your rod width. However, you should make sure that there aren't any obstacles near your window that will interfere with your finial or end cap choice. If your window is near a side wall and you don't have much room for a finial, it's become popular to use an end cap on the end of the drapery rod nearest the wall and a finial on the other end. Using both a finial and end cap can add extra customization to your window treatment.
The amount of rod that you chose to extend past the window or window trim is totally up to you. Once you've examined your drapery style and accounted for obstacles, your finial rod size is really a matter of your personal taste.