Does the calendar fill up the whole screen automatically when I rotate my iPad?
No it doesn't. You can set the screen to portrait or landscape mode only once when you first create your planner to avoid mislocations of your written data.
If you want to change the size of your calendar, please create a new one on the Management page and select the screen orientation.
*When you delete the old one, please note that your written data and items will be deleted too.