Never mind. I thought I exhausted all my options but right after I posted (of course) I figured it out.
In case anyone else has the same question:
By default the text size inside the tabs was set to 13. So I went into Shortcodes>Settings from the dashboard, and this is the bit you’re looking for to put in your Custom CSS pane:
.su-tabs-pane {
padding: 15px;
font-size: 13px;
-webkit-border-bottom-right-radius: 3px;
-moz-border-radius-bottomright: 3px;
border-bottom-right-radius: 3px;
-webkit-border-bottom-left-radius: 3px;
-moz-border-radius-bottomleft: 3px;
border-bottom-left-radius: 3px;
background: #fff;
color: #333;
}