First you have to decide on a page size for your book. Then you need to know the book's page count. Armed with this information, you can obtain a free template through Ingram: https://www1.ingramspark.com/Tools/CoverTemplateGenerator.
The template will show you the bleeds and the width of the spine.
Here is one question which addresses how to create a book cover solely using LaTex: Create a book cover with spine, rear page and flaps
Your cover will be a different size than your text block, because the cover wraps around and encloses the text block. You might consider a graphics program (free: Gimp or Scribus, proprietary: Photoshop or InDesign) to create the cover once you know the dimensions. There are specialized programs for book cover creation as well, like Book Cover Pro.
If you are using POD printers such as Lulu or Createspace, these companies have free cover generator programs which can be used when you avail yourself of their printing services.
Since you already have the cover in Photoshop, use LaTex to typeset the text block of your book. Memoir is perfect for this, yet it is not your only choice. There are other book options (e.g., book, Komascript, tufte-book, book-est, etc.). Create a pdf file with LaTex and then send this file with your Photoshop cover to the printer of your choice.
January, 2018 edit: I wrote this answer four years ago. There is now a wonderful LaTeX package for book covers, called, unsurprisingly, bookcover. It is easy to use, lets you specify dimensions, drop in your front cover and back cover art, generate CMYK output for printers, rotate text for the spine, etc. Before this package, putting together a book cover required an external program like the Gimp, Krita, InDesign, Quark or any of their competitors. Now putting together a book cover is trivial. Outputting book covers and fighting with printing templates used to be one of the worst parts of indie publishing. With this package, it's no longer an issue.