how to chage background size in css code example

Example 1: background image size css

#example1 {
  background: url(mountain.jpg);
  background-repeat: no-repeat;
  background-size: auto;
}

#example2 {
  background: url(mountain.jpg);
  background-repeat: no-repeat;
  background-size: 300px 100px;
}

Example 2: how to set background automatically with my screen height

body {

    background-image: url("img_tree.gif");

   background-repeat: no-repeat;   
 background-attachment: fixed;
  
   background-position: top center;
  background-size: contain;

 }