As near as I can tell the size is set automatically based on the size of the tables (how much space and overall dimensions compared to the full sheet). I took the invoice template and played around. As I shrank the tables and moved all the elements into a smaller space the page sizes kept shrinking.
It is a matter of experimenting. Be sure to make sure all elements are reduced and the overall space taken by all elements smaller. The text box at the bottom is easy to overlook because the actual text takes up a very small amount of space, but when you see the entire text box, it is quite large.
Alternately, you can print the page using a utility like Printopia (for the Mac) or FingerPrint (form Mac or Windows). These utilities allow you to print as a PDF to a folder instead of to the printer. The Print dialog for Numbers lets you adjust between landscape and portrait, and adjust/resize the sheet to print to a single or multiple pages.
On my iPad this is a standard letter sized format 8.5 x 11.5. I'm thinking that if your overall regional settings on the iPad are U.K. this might change to A4. Not sure.
I could find no way to set the page size other than these options. Doesn't mean it doesn't exist, just that I can't find it if it does.
