I'm not aware of any way to enlarge the book cover unless you're in the book itself. If you have the book open and press Menu, then "Go to," and select "cover," that's about the largest version you're going to see on the Kindle.
You mention "prior to buying the book," so I'm not sure if that helps you. If the publisher included the cover in the e-book (which they should), then you should be able to view the cover in the downloaded e-book sample using the method above.
I hope that answers your question adequately. If not, just let us know.
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