background-size: auto (default):

Hello W

Hello World

Here, the background image is set to 300px wide and 100px high.

Hello World

Here, the background image is t to 300px wide and 100px h

orld

Here, the background image is set to 300px wide and 100px high.

background-size: 300px 100px:

Hello World

Here, the background image is set to 300px wide and 100px high.

Note: The background-size property is not supported in Internet Explorer 8 and earlier versions.