As an ebook, you can choose any size and layout you desire but there are some best practice.
For landscape orientation, 12 width x 9 height inches (4:3 aspect ratio) will give you the best all round fit for both digital devices (tablet, phone, kindle and if you want to get it printed via a service like Kindle Direct Publishing you can just split the pages into 2 by dividing the width (6 x 9 inches – 2:3 aspect ratio). You can adjust the layout to portrait orientation (9 x 12 inches) but keep in mind it’ll be harder to split the pages if you want to print on Kindle Direct Publishing.
As a generat rule of thumb:
Printed Books (Portrait standard): 6 x 9 inches / 2:3 aspect ratio
Kindle eBook (Landscape): 2560 x 1600 px / 16:10 aspect ratio
iPad (Landscape): 2048×1536 px / 4:3 aspect ratio
Using the information above, our sweetspot is 12 x 9 inches / 2000 x 1500 px / 4:3 aspect ratio.
On our website, we require a cover image of 6 x 9 inches / 1000 x 1500 px / 2:3 aspect ratio for cosmetic reasons.