Is this a file that is hosted over a network?
Hosted by server or by a copy of FileMaker Pro?
It is hosted over a network by a copy of FMP.
See if you can use printer set up to change the page size on the host machine rather than from a client. I think you'd have to do this once for every layout used for printing. This should set up a "default" paper size. Users can still change this in printer setup, but the changes from client machines won't persist.