I would like to know how to set the app up to automatically download the full email message.

You can change the email download size with the following steps.

Please set the settings as 'All' and whole email will be downloaded automatically.

- Nine Settings > Select your account > Content to sync / Email > Email download size

If you can't see the 'All' option in the settings, you have the email download size limitation policy on server.

Please contact your server admin to disable the policy on server.