First, you must be logged in as the Admin since only your school’s Admin can change these options. If you do not see Admin in the top-level toolbar, shown in the screenshot below, then you are not logged in as the Admin.
Once you are logged in as the Admin, click on Admin in the top-level toolbar. Then click on the Design Settings button in the second-level toolbar.
From here, you can adjust the global portrait settings for the entire book.
Please note that you can change the font and font size, but you cannot do so on a flow by flow basis. Once you have made your changes, press Save.