Cover Dimensions and Spine Size Calculator can be found HERE. Things to keep in mind when building your soft cover: It's very easy to get "lost" in all this design jargon about bleeds and safe zones so let's take a minute to visualize what your file will look like when it's a finished cover.
Basic Layout for Soft Cover:
The Finished Soft Cover
Cover file with bleeds, safe zones, and your spine
The finished book. Notice all the bleed has been cut off