All of the Templates provided in ages for iPad are Letter Size Portrait Documents. To create something custom, it is best to create it in Pages for Mac and then sync it to your iPad. A perfect example is Landscape documents. There are none in Pages for iPad, so I had to sync one to my device, and then duplicate it as needed.
In iOS 7 and using the newest version of the iOS and OS X iWorks apps it's even easier to create and use different sizes than it was in iOS 6. Files are now completely compatible. If you're using iCloud to sync you don't even need a method to send the new page size to the iPad.
You do have one choice besides Letter Size in iOS. The A4 size is a bit smaller, and the standard in many countries outside the U.S.
Of course, this still doesn't help those who don't own a Mac. They will still have to hunt for templates online or ask a Mac owner for help.