psnupのデフォルトの紙サイズ
psnupのマニュアルによると、
The -p option can be used as an alternative, to set the paper size
to a0, a1, a2, a3, a4, a5, b5, letter, legal, tabloid, statement,
executive, folio, quarto or 10x14. The default paper size is
normally a4, but on a Debian system, /etc/papersize is consulted.
ということで/etc/papersizeを
letter
に変更。