Edit in JSFiddle

CSS
HTML
Result

div {
  height: 100px;
  width: 200px;
  background-image :url('http://ezcode.kr/study/css/img/smile.png');
  background-repeat: no-repeat;
  border:1px solid black;
}
.ex1 {
  background-size: auto;
}
.ex2 {
  background-size: 20px 75px;
}
.ex3 {
  background-size: cover;
}
.ex4 {
  background-size: contain;
}

<!DOCTYPE html>
<html lang="ko">
  <head></head>
  <body>
    <h1>background-size</h1>
    <p>background-size: auto;</p>
    <div class="ex1"></div>
    <hr>
    <p>background-size: 20px 75px;</p>
    <div class="ex2"></div>
    <hr>
    <p>background-size: cover;</p>
    <div class="ex3"></div>
    <hr>
    <p>background-size: contain;</p>
    <div class="ex4"></div>
  </body>
</html>