Although I'm sure there must be an easier solution, several online
sources suggest that recreating the user profile solves the problem. One
way of doing this (I assume) is given at
http://www.omnitechsupport.com/forum/topic/169/when-try-to-print-envelope-get-message-the-number-must-b/
Some previous replies say that this error can occur if you have
added a header or footer to the Normal template, but other users report
that this occurs in a "virgin" install of Word 2007.
Another suggestion that is really only a workaround is to use
envelope templates rather than the Envelope dialog; for some examples,
see
http://www.gmayor.com/downloads.htm for some downloadable templates.
FWIW, the "between 1 and 31 (points)" measurement usually refers to
the "distance from text" of a border or page border, and I believe this
error message is a rather generic one that we have seen in the past in
other unrelated situations as well.