The trim size of your book dictates approximately how many words fit on one page, so start by choosing a trim size. (bookdesignmadesimple.com)
Not done on pages because that's so dependent on size of book, type - face, and of course no relevance in ebooks. (writerswin.com)
NOTE: Although readers can display higher resolutions that usually increases the file size of the book, which isn't necessary (not for a book cover). (unrulyguides.com)