Where can I change the font size for the message on the landing page after clients opt-in for my autoresponder?

This is something you can modify from the Console's website templates. You can go to WEBSITE TAB > Theme, click on the "PREVIEW & CUSTOMIZE" button and, in the website template customizing window, modify the font size and color in the section to the left. This will be applied to all the website pages, including the message in the default landing page the Coaches Console has already set up for your clients to see after they subscribe to your autoresponder.