When using the FreeTextBox HTML editor, the default font styles have to be set in the stylesheet specified in the DesignModeCss property.

In my case I created a stylesheet and saved it as /App_Themes/Default/FreeTextBox.css, with the following:
body
{
font-family:Georgia;
font-size:14pt;
}
Then I set the DesignModeCss property:
<uc:FreeTextBox ID="ucFreeTextBox" DesignModeCss = "~/App_Themes/Default/FreeTextBox.css" Runat="server" />
And that’s it.