How can I change the calendar size?

You can select the format of the calendar by clicking on the icons showing the different formats underneath the design on the design page (the page where you select the calendar you want to create).
Once you have begun creating the calendar, it is unfortunately not possible to change the format/size, and you must go back to the design page to select the format. It is also not possible to change the format/size of the calendar that you already made. If you want to reorder a calendar and want it in a different format/size, you need to create a new calendar back from scratch and choose the format/size you want for the calendar.