create a blog to a full background

Go to "EDIT HTML"
 find this code:
 body { font: $(body.font); color: $(body.text.color); background: $(body.background); }

 enter the red code below: 

 body{
      background-attachment:fixed;
      background-size:cover;
     filter: progid:DXImageTransform.Microsoft.AlphaImageLoader

(src='bg-body.jpg', sizingMethod='scale'); /*IE 8 */
     -ms-filter: "progid:DXImageTransform.Microsoft.AlphaImageLoader

(src='bg-body.jpg', sizingMethod='scale')";  /*IE 8 */
} 

save.

you can create your own size by entering the code below:

body{
     background-attachment:fixed;
     background-size:1024px auto;
     filter: progid:DXImageTransform.Microsoft.AlphaImageLoader

(src='bg-body.jpg', sizingMethod='scale'); /*IE 8 */
     -ms-filter: "progid:DXImageTransform.Microsoft.AlphaImageLoader

(src='bg-body.jpg', sizingMethod='scale')";  /*IE 8 */
}  

yellow to change the size, 1024px