If you now how many pages wide and tall your diagram should be
- Select File->Page Setup... from the menu
- Select "Fit to"
- Enter the number of pages
Are you aware that "page breaks" are always displayed in Dia (usually blue lines) That's an easy way to check how your printed diagram will end up on the pages
If the simple approach with the number of pages does not fit your approach, you'll have to switch back from "Fit to" to "Scale" and work out the scale that you're looking for.