The standard email templates that come with the plugin have been tested millions of times by thousands of stores. Their simple layout provide excellent review conversion rates. We do not recommend changing font size but it is possible to do it.
Visual WordPress text editor does not have a button for changing font size. However, you can customize text size by switching the editor to 'Text' mode and adding inline CSS rules.
Add an HTML element <div> with inline CSS style like shown below in the beginning of the message:
<div style="font-size: 18px;line-height: 28px">
And a closing HTML element </div> at the end of the message like this:
If you want, you can switch back to the 'Visual' mode after adding inline CSS.
The possibilities of inline CSS are not limited to just changing font size. You can search for additional information about using inline CSS in the Internet.