(La)TeX PDF output prints at wrong size

Having got everything else right, you should be aware that the problem may have nothing to do with (La)TeX and everything to do with the program you use for printing. A regular cause for such problems lies with Acrobat Reader, which by default enables its option to scale pages to fit on the printable area of the paper. Since a printer can rarely print right to the edge, this means that pdf-files will be shrunk by some (small) factor (even if the pdf-file is formatted for A4, and your paper size is set to A4 as well).

Correcting this silliness is not very hard, but the exact details depend on the version of Acrobat Reader (or "Adobe Reader" from version 6.0 onwards) you have installed:

• Mac OS X, Adobe Reader 6:
  in the print dialogue, on the "copies & pages" pane, you'll find a popup menu titled "Page Scaling". Make sure that the menu reads "None".
• Windows, Adobe Reader 6:
  in the print dialogue, select "None" from the drop-down list "Page Scaling".
• Windows, Linux Acrobat (Reader) 5.0:
  In the print dialog, make sure the "Shrink oversized pages to fit" checkbox is unchecked. It may also be useful to uncheck the "Expand small pages to fit paper size" checkbox as well.

This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=outszwrng