There is a quick way to change the page size of all of your parts in one edit operation within Sibelius. Here’s how:
In Sibelius 7 or later, locate the Layout Group in the Parts Tab and select All Parts:
(In Sibelius 6 choose Multi-Part Appearance button at the bottom of the Parts Window)
Select your new global parts page size:
Don’t forget to change the printer Page Setup… size to match this new size for when you print.
That’s all there is to it.