To be clear, you're asking about resizing a pdf, not a jpeg, right? Jpegs you can resize in any decent photo software. Your first paragraph makes me think you mean the whole pdf cover, though you only mention jpegs in the last sentence.
If you're trying to resize a pdf paperback cover, I don't think that can be done. When I searched for resizing pdfs, all I got was how to change the file size, not the physical dimensions. You can crop a pdf, but that's not what you want.
Best bets (in order of easiness):
First choice--wait for a definitive response from Amazon. You just got the generic response. That doesn't mean Amazon can't do it; it just means the first-level responder can't do it. Chances are you can get the problem fixed without resubmitting anything. The linking issue has nothing to do with your files. If, as you say, it's the description, you can change that if needed without reuploading anything.
Second choice--contact the cover designer for a revised cover. Typically, designers will do this for you for free, but it might be worth paying a small fee if you absolutely must have a new file.
Third choice--(requires Acrobat, which I'm not sure you have) Follow the directions under "Export Images to Another Format"
https://helpx.adobe.com/acrobat/using/exporting-pdfs-file-formats.html That lets you extract the image from the file and resize it as needed.
Don't overcomplicate the problem. The resolution may be simpler than you think.