The width of the spine depends on how many pages your book has - it's simple but the more pages, the thicker the spine needs to be to wrap round them all. Best not to design the cover layout before you have finished your inside page artwork because if the number of pages changes the spine will get bigger or smaller.
We have listed a few of the common spine sizes here to give you an idea of your book's thickness. If your number of pages or stock isn't listed then drop us an email at email@example.com
and we will be happy to advise on the correct size.