Most 5" and 6" readers have a 600x800 pixel screen, unfortunately not all use the full size for displaying books. My Nook Classic for example has only 600x730 pixels usable for display after removing the space used for the top and bottom bars. I don't know if anyone has made a list of the usable screen size of all of the readers. I usually limit my inside images to 600x730 pixels and use and use this xhtml code for the 600x800 pixel cover.
Code:
<?xml version="1.0" encoding="utf-8" standalone="no"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN"
"http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<title></title>
<style type="text/css">
/*<![CDATA[*/
@page{padding:0pt;margin:0pt}
body{oeb-column-number:1;text-align:center;padding:0pt;margin:0pt;background-color:#ffffff;}
/*]]>*/
</style>
</head>
<body>
<div>
<svg xmlns="http://www.w3.org/2000/svg" height="100%" preserveAspectRatio="xMidYMid meet" version="1.1" viewBox="0 0 600 800" xmlns:xlink="http://www.w3.org/1999/xlink">
<image height="800" width="600" xlink:href="../Images/Cover.jpg"></image>
</svg>
</div>
</body>
</html>
This allows the cover image to scale to the screen size of the device.
Hope this helps.
JSWolf beat me to it.