Your book cover is actually entirely separate from Atticus, so you'll have to do a bit of calculation.
What you will need from Atticus is the trim size of your book and the total page count. To find the total page count, open your book in Atticus and click on Formatting from the center, you can change the preview to Print by clicking the dropdown above the preview (it defaults to iPad) and change to Print. Give it a moment to render your book and the page number will appear in parentheses below the previewer.
You can then take the trim size and page count to your publisher/distributor, and they should have a calculator for you to use. For example,