How to resize a background image using CSS? code example

Example: how to change the width of a background image in css

#example1 {		/* Example1 has an auto background size */
  background: url(mountain.jpg);
  background-repeat: no-repeat;
  background-size: auto;
}

#example2 {		/* Example2 has an fixed background size */
  background: url(mountain.jpg);
  background-repeat: no-repeat;
  background-size: 300px 100px;
}