No problem!

There is no UI control in the Embed Theme Builder for font-size, but there is the ability to link up Custom CSS, so that's the feature we'll use here. It gives you 100% control over the CSS of your Embedded Pen.

Now that we've created this new theme (remember CodePen PRO members get unlimited themes) we can set it up how we like. Feel free to pick colors and such, but the big thing here is dropping in a URL to a CSS file.

The trick here is that you can use CodePen itself to host that CSS. Here's an example of a Pen, and the URL format directly to that Pen's CSS:

codepen.io/chriscoyier/pen/OgEdqp
codepen.io/chriscoyier/pen/OgEdqp.css

Now that's the URL we'll pop into the Custom CSS field as part of the Embed Theme. The Custom CSS will apply and the font will be bigger!

The CSS I used there is:

#output pre {
  font-size: 18px;
}

Which overrides the default that we've set of 13px.

Remember this isn't just for a single Embedded Pen. We've created a theme here, so you can apply it to as many Embeds as you like, and it makes updating the style across all of them easy.