Is it possible to define the resultant page size when a single topic is "Saved as PDF"? The default seems to be A4 and I can't see any way to change it.
Assuming you mean in the output, I think it's controlled by the user's first listed "Preferred languages" (in Windows - I assume there's similar for Mac and Linux). But if they choose a different option I think that is remembered until they change their preferred language again. If may also depend on the browser, because FF seems to pick the values defined for the specific printer, so the user would have to change those, themselves.