Great sunsets are one of my favorite subjects. The longer the focal length of the lens you are using, the less area of colorful sky you need to have for a great picture. In other words, a long focal length can capture a great picture of small sunset display. The sunset in this picture was incredible regardless of the focal length I used.
Here is a tip for you: If one of your borders is a solid color (preferably black or white), you can easily add to it later in post processing. Here, I framed the picture so that I had just enough black on the bottom of the frame to allow more black base to be added later in Photoshop. Thus, I get a higher resolution final image with very little work.
Camera and Lens Settings
300mm f/5.6 1/100s
5616 x 3964px