Here are the cover size specifications for some major ebook retailers:

1. Amazon Kindle: For Kindle ebooks, Amazon recommends a cover size of 2560 x 1600 pixels, with a 1.6:1 aspect ratio. The maximum file size is 50MB.
2. Apple iBooks: Apple recommends a cover size of 1400 x 1873 pixels, with a 1.34:1 aspect ratio. The maximum file size is 10MB.
3. Barnes & Noble Nook: For Nook ebooks, Barnes & Noble recommends a cover size of 1400 x 2100 pixels, with a 2:3 aspect ratio. The maximum file size is 5MB.
4. Kobo: Kobo recommends a cover size of 1600 x 2400 pixels, with a 2:3 aspect ratio. The maximum file size is 2MB.
5. Google Play Books: Google recommends a cover size of 1600 x 2560 pixels, with a 1.6:1 aspect ratio. The maximum file size is 20MB.

It’s worth noting that while these are the recommended cover sizes for each retailer, there may be some variation in the specific requirements depending on the device or platform used to view the ebook. Additionally, it’s always a good idea to review each retailer’s specific guidelines to ensure that your cover meets their requirements.