An important part of designing your very own book cover is knowing the size of things.
Are you publishing on kindle ebook format?
Or are you mocking up some covers for traditional publishing?
Or are you doing it for fun, to imagine what your final product might look like?
The Generic ratio is 1.6:1. This means that if you did some mathy-stuff, the length of the tall side would be 1.6 times longer than the width of the short side.
A good rule of thumb is to design the spine of the book at about an inch.
The back cover is often not designed like the front, because it will have a body of text (back of the book description of the story). Yet it should have the same dimensions as the front cover.
From Amazon Kindle’s public webpage:
• A minimum of 625 pixels on the shortest side and 1000 pixels on the longest side
• For best quality, your image would be 2820 pixels on the shortest side and 4500 pixels on the longest side
Actual Publishing Covers:
If you are involved in designing the cover for your book for the actual publication, you’ll have to consider all sorts of variables from paperback vs. hardcover, to the individual height and width.
Essentially, this is information you’ll have to get from your publisher/editor. And once you have the dimensions, they’ll probably have suggestions or requirements for what format and program you use.
If you’re just putzing around in the same way that I like to, then you can just use the generic dimension ratios.