book will be published by 48hourbooks.com. They have recommeneded that the cover be slightly larger than the book itself, to account for bleed (when the printing goes all the way to the edge). Since it's a wrap-around cover, it needs to be TWICE the width of the finished book, plus the spine. To determine the width of your spine, divide number of Pages (not SHEETS, but PAGES) by 440. For example, if your book was 120 pages, the spine would be 120 / 440 = 0.27 inches. My book is 71 pages.
So for a 5.5 x 8.5 book that is 120 pages long, the cover should be 8.5 tall, and 11.27 wide (5.5 front + 5.5 back + .27 spine = 11.27). And for a 8.5 x 11 book that is 200 pages long, the cover should be 11 tall, and 17.45 wide (8.5 front + 8.5 back + .45 spine = 17.45). However, my book will be 6x9.
Of course, if your artwork is going to BLEED (go all the way to the edge), you'll need to have it extend 1/8" BEYOND the edges on top, bottom and sides.
Include the title and author name on the spine so when your book is on a bookshelf, the title and author name read from top to bottom.