I’ve had a few queries about this, so I’ll post a couple of examples here to get us going:

To change page size without scaling the contents:

An ideal example is to change from US Letter size to A4 size, since usually the contents will easily fit the new dimensions:

-Open a US Letter sized document.
-Open the document settings panel. [File→Document Settings… ⌃⌥⌘D]
-Select the “Size/Margins” tab.
-From the “Page Size” popup menu select “Fixed Size…”.
-In the Page Setup dialog select A4 as the Paper Size and click OK.
-From the alignment pop menu select “Center”.
-Exit the Document Settings panel by clicking OK.
-Export to a new PDF document. [File→Export to PDF→Single File… ⌃⌘S]

From US Letter size to 6in x 9in size:

-Open a US Letter sized document.
-Open the document settings panel. [File→Document Settings… ⌃⌥⌘D]
-Select the “Size/Margins” tab.
-From the “Page Size” popup menu select “Fixed Size…”.
In the Page Setup dialog select you need to create the 6”” x 9”” paper size:
-From the “Paper Size” popup menu select “Manage Custom Sizes”.
-In the “Custom Page Sizes” dialog create a new page size by clicking the “+” button, and name it appropriately (double click the new size in the list to change its name).
-Set the page size width to 6 inches and the height to 9 inches.
-Set all margins to 0 (for clarity) and click OK.
-In the Page Setup dialog select the new page size from the “Paper Size” popup menu and click OK.
-From the “Alignment” pop menu in the Document Settings panel select “Center”.
-Exit the Document Settings panel by clicking OK.
-Export to a new PDF document. [File→Export to PDF→Single File… ⌃⌘S]