There are two ways to adjust the size of text inside of the WYSIWYG editor. 

1. The more general option is to select the text and assign it a preset size from the formatting drop-down: 

2. You can edit text sizes in the WYSIWYG source code-- second to last menu button on the WYSIWYG menu: 

Everything is written in the source code is in HTML whereas font sizes are a CSS feature. Therefore, you will need to format the CSS in HTML in order to make the change. Any CSS component formatted in HTML begins with a 'style' tag.

Say you are looking to change this paragraph to a 15px font size:

<p>This is a paragraph</p>

Since the paragraph above doesn't yet have a style tag, you will need to add it as well as the font size:

<p style="font-size: 15px;">This is a paragraph</p>

Some of the other elements of your page already have some styling (i.e. being centered on the page):

<h3 style="text-align: center;"><strong>Downtown Los Angeles</strong></h3>

At this point, all you will need to add is the font size:

<h3 style="text-align: center; font-size: 15px;"><strong>Downtown Los Angeles</strong></h3>

Finally, if the text you are looking to change already has a font size attached, you will need to only adjust the pixel limit:

<h4 style="text-align: center; font-size: 15px;"><strong>Downtown Los Angeles</strong></h4>

to

<h4 style="text-align: center; font-size: 20px;"><strong>The Vortex - Los Angeles</strong></h4>

Finally, text sizes can be changed for your website in the theme files. You can read more on that here.