I use width="100%" and let the height take care of itself. It works on the Sigil display, on all Kindle devices and apps, on the Nook, and on ADE. (In my experience, I hasten to add, in hopes of avoiding another debate. I am only saying what works for me.)
That said, my first impulse was to copy Sigil's cover html, but I see Toxaris has already suggested that. Here it is in any event:
<div style="text-align: center; padding: 0pt; margin: 0pt;">
<svg xmlns="http://www.w3.org/2000/svg" height="100%" preserveAspectRatio="xMidYMid meet" version="1.1" viewBox="0 0 800 1169" width="100%" xmlns:xlink="http://www.w3.org/1999/xlink"><image height="1169" width="800" xlink:href="../Images/coverepub.jpg"/></svg>
</div>