Check out example codes for "how to change background image size in css". It will help you in understanding the concepts better.

Code Example 1

#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;
}

Code Example 2

background-size: 100px 50px; /* 100px wide, 50px tall */

Code Example 3

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

    }

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