Is this for a Kindle eBook?
I found https://kdp.amazon.com/en_US/help/topic/G200645690 which says:
- Ideal dimensions for cover files are 2,560 pixels in height x 1,600 pixels in width. For best quality, particularly on high definition devices, your image should be 2500 pixels in height.
I also found a cover creator, which I presume is what you are using. But it doesn’t seem to be possible to try that out without first creating a book to attach the cover to.