how to set the body size in css code example

Example 1: css size

#myID {
  height: 100px; /* Sets the height */
  width: 200px; /* Sets the width */
}
/* or... */
.myClass {
  height: 220px; /* Sets the height */
  width: 40px; /* Sets the width */
}
/* They both work! */

Example 2: fixed size body html

html {
    position: relative;
    min-width: 1024px;
    min-height: 768px;
    height: 100%;
}
<!-- Remove height 100% if needed fixed height -->