This is a Photoshop action set that comes with 14 actions that make skies out of nothing.
It is super-easy to use and it comes with instructions, along with tips and tricks that will help you create an infinite number of new skies.
You can choose the size for your sky, so it will fit in your design perfectly. The fact that these are actions and not bitmap textures makes it possible for you to choose your image size and resolution.
Each time you run the same action, you’ll get a similar, but different sky.