You can use a built-in WYSIWYG editor to change the font size:
Or you can change the source code:
This is a Zendesk huge size or Header 2
<p><span class="wysiwyg-font-size-x-large">This is a Zendesk huge size or Header 2 </span></p>
<h2>This is a header 2</h2>
This is a Zendesk large size or Header 3
<p><span class="wysiwyg-font-size-large">This is a Zendesk large size or Header 3</span></p>
<h3>This is a header 3</h3>
This is a Zendesk medium size or Header 4
<p><span class="wysiwyg-font-size-medium">This is a Zendesk medium size or Header 4</span></p>
<h4>This is a header 4</h4>
This is a header 5
<h5>This is a header 5</h5>
Small size
<p><span class="wysiwyg-font-size-small">Small size</span></p>
This is normal text. Lorem ipsum is a pseudo-Latin text used in web design, typography, layout, and printing in place of English to emphasize design elements over content. It's also called placeholder (or filler) text.
<p>This is normal text. Lorem ipsum is a pseudo-Latin text used in web design, typography, layout, and printing in place of English to emphasize design elements over content. It's also called placeholder (or filler) text.</p>