Here's the code I use in the titlepage.xhtml (or the cover HTML file, whatever name was assigned to it) when the image is not the correct size:
<?xml version='1.0' encoding='utf-8'?>
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en">
<head>
<meta name="calibre:cover" content="true"/>
<title>Cover</title>
<style type="text/css" title="override_css">
@page {
padding: 0;
margin: 0;
}
body {
text-align: center;
padding: 0;
margin: 0;
}
div {
padding: 0;
margin: 0;
}
img {
padding: 0;
margin: 0;
}
</style>
</head>
<body>
<div>
<img src="cover.jpeg" alt="cover" style="height: 100%"/>
</div>
</body>
</html>
Copy and paste all of the codes above and give it a try. Replace "cover.jpeg" with the name of the cover image file.