HTML:

<div id="element-one">Hello I am some text.</div>
<div id="element-two">Hello I am some smaller text.</div>

CSS:

#element-one {
    font-size: 30px;   
}

#element-two {
    font-size: 10px;
}

The text inside #element-one will be 30px in size, while the text in #element-two will be 10px in size.