Based upon Publisher 2007 but it should be the same in 2010:
Open your template.
Click 'Calendar Options'
Click 'Change Page Size'
Change the height and width to your desired measurements.
Click 'OK'
You'll now see the A4 size design laid on top of your smaller page. So you need to resize the design, as follows:
Position your mouse cursor beyond the top-left of the design.
Click and drag down to beyond the bottom-right of the design.
(That will select all objects on the page)
Go to the 'Arrange' menu, and click on 'Group.
The design will now be a single object.
Use the 'handles', at the corners, to get the design to fit the page.
Go to the 'Arrange' menu, and click on 'Ungroup'.
Modify the template to meet your needs.
Chris