Scales an image multiple times in defined step sizes until the desired size is reached while retaining the aspect ratio of the image.
This is basically the "Scale in steps" script from K.-M. Hansche tailored to my needs.
Give the desired size of the image and select whether this is for width or height. The step size defines how much pixels the image will be shrunken every time. When flattening the image, the layers will be merged and the image is automatically cropped. The other options control the scaling itself.