Unless your cover has a white background, the size of a cover needs to expand to accomodate bleeds, hinges, and wraps. Click one of the cover styles on the left to create a PDF diagram displaying dimensions for your cover.
Adobe Acrobat or Adobe Reader.
You can get Adobe Reader for free.
Click here to get the reader.
Notice for Mac users: OS-X Preview displays blank values. Save to disk and open with Acrobat.