To change the size of your subscription, please do the following:
From your Chatbooks app:
Tap on your subscription
Tap the settings wheel in the upper right corner of the screen
Tap Book Options and select 6x6 softcover or hardcover
Tap Save and Done
From the web at chatbooks.com:
Click on your Bookshelf
Click on your subscription
Click the Settings wheel in the upper right corner
Click Book options
Under Book Size, select 6x6
Under Cover Type, select Softcover or Hardcover
Under Cover Color, you can keep what is currently shown or select a different color
Once you click the back arrow by Book Options, your changes will be saved.
Your new price will reflect on your next billing cycle.