If you want to control the size of the content box of a block element, you can use the "width" and "height" properties as shown below:

* {width: 300px} - Specifies the content box to be 300px wide.

* {height: 200px} - Specifies the content box to be 200px high.

The HTML and CSS document below shows you a good example of how to specify content box size of on a <P> tag: