Here is a template that will help you perfect your 6×9 full-cover design for your paperback book. This is a 2-part overlay: front and back with bleeds and margins, including spine margin and the default barcode size and placement.
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This online calculator tool will help you arrive at the dimensions your full cover needs to be for Amazon KDP or for CreateSpace or you can just use it to figure your spine width. Different printers use different paper, which determines the spine width, which is why this calculator is specific to Amazon KDP and CreateSpace (same paper).