If you use your web-browser's builtin Web Developer Tools to Inspect the HTML Elments of the current Passage of a story while you play it, and then select each of the main elements in turn (like html, body, tw-story, tw-passage, etc...) the Styles section will display what CSS is effecting that element.
If you look at the Styles section while the tw-story element is selected you will see that the following CSS is being directly applyed to it.
tw-story {
display: -webkit-box;
display: -webkit-flex;
display: -moz-flex;
display: -ms-flexbox;
display: flex;
-webkit-box-direction: normal;
-webkit-box-orient: vertical;
-webkit-flex-direction: column;
-moz-flex-direction: column;
-ms-flex-direction: column;
flex-direction: column;
font: 100% Georgia, serif;
box-sizing: border-box;
width: 100%;
min-height: 100%;
font-size: 1.5em;
line-height: 1.5em;
padding: 5% 20%;
margin: 0;
background-color: #000;
color: #fff;
}
The font: 100% Georgia, serif; setting in the above lets us know that Harlowe's default font-family is Georgia and that the default font-size starts off as 100% of whatever font size that the web-browser your using defaults too.
The later font-size: 1.5em; setting override lets us know that Harlowe's default font-size is 1.5 times the font size of the previous value which was 100% of the web-browser's default font size.
So if your web-browser's default font size is 16px then Harlowe's default font size will be 24px.
(16 x 100%) x 1.5 => 16 x 1.5 => 24
... however if the default font size of your web-browser is some other value then Harlowe's default will also be different.