Final answer:
The property of the CSS rule is 'font-size' and it sets the size of the text within an element.
Step-by-step explanation:
The property of the following CSS rule is font-size. In CSS, properties define the characteristics of elements on a webpage. The font-size property specifically sets the size of the text within an element. For example, if you set the font-size property to 40px, then the text within that element will be displayed at a size of 40 pixels.