HTML/CSS - Automatically adjust size of element for different screen size(s)

You can set your width using relative percentage, such as 50%, the actual width will be 50% of the parent width. To center the absolute header, you can set its left and right to 0 and margin:0 auto:

#header{
  position: absolute;
  top: 10%;  
  left:0;
  right:0;
  margin:0 auto;
  width: 50%;
  height: 100px;
  border:thin solid black;
  text-overflow:ellipsis;
  overflow:hidden;
}