For your eBook cover image file size, please ensure you follow the guidelines given on the publishing platform of your choosing. The cover shown within the Atticus previewer may appear slightly different depending on the platform and image size.
KDP eBook Cover Guidelines: https://kdp.amazon.com/en_US/help/topic/G200645690
Apple Books Cover Guidelines: https://help.apple.com/itc/booksassetguide/en.lproj/itc1bda991ba.html
Ingram Spark Cover Guidelines:
https://www.ingramspark.com/plan-your-book/ebooks