That its the correct code of background size :
<div class="html-mobile-background">
</div>
<style type="text/css">
html {
/* Whatever you want */
}
.html-mobile-background {
position: fixed;
z-index: -1;
top: 0;
left: 0;
width: 100%;
height: 100%; /* To compensate for mobile browser address bar space */
background: url(YOUR BACKGROUND URL HERE) no-repeat;
center center fixed;
-webkit-background-size: cover;
-moz-background-size: cover;
-o-background-size: cover;
background-size: cover;
background-size: 100% 100%
}
</style>