Apply the style element, i.e. ‘font-size’.
In text mode you could surround your text with any element, for example a div or paragraph tag and put the style in there, such as:
<div style="font-size: 16px ;">
<p>text text text text</p>
</div>
Or when you are editing in the widget, on the right under Attributes, you can place the css code straight into the ‘css styles’ box, e.g. font-size: 16px ;
ALternatively, create a class, say ‘text-size16’ and place that inside an element pair:
WIDGET
<div class="text-size16">
<p>text text text text</p>
</div>
CSS FILE
.text-size16 { font-size: 16px ;}
That’s how I do it anyway.