Why is the standard size for a sheet of paper 8.5" x 11"? Please don't
say it is so it fits in a copy machine! Thanks!

I've found quite a bit of information on this intriguing question,
which was "Question of the Month" in the current e-newsletter of
"Paper University."

Question: Why do we use 8 1/2 x 11 inch paper as our standard letter
size?

Answer: Standardized paper sizes actually came about as a more-or-less
hit-or-miss situation, rather than a planned event. Here's the whole
story of paper sizes:

The quest for standardized paper sizes began in the 14th century in
Bologna, Italy. In the year 1398, a marble tablet inscribed with the
outlines of four sizes of paper [small, medium, large, and
extra-large] was placed in a public place to serve as a guide for the
sizes of paper manufactured in that region of Italy. This appears to
be the first time that paper sizes were regulated and standardized.

Centuries later, in 1786, physics professor Georg Christoph
Lichtenberg of Germany noted the advantages of paper sizes having a
height-to-width ratio of the square root of two (1:1.4142). The
Lichtenberg Ratio has the advantage of preserving the aspect ratio
when cutting a page in two. This ratio is also the basis for the
metric-based ISO paper size system used by most of the industrialized
nations today.

A few years later, in 1794, the French government issued a law that
specified paper size formats that correspond exactly to several of the
modern ISO paper sizes. Today the United States and Canada are the
only modern countries in which the ISO standard paper sizes are not
widely used.

The historic origins of the U.S. letter size format (8½ x 11" / 216 x
279mm) are relatively obscure. There were attempts in 1921 by the
Permanent Conference on Printing, and also by the U.S. Secretary of
Commerce, former President Hoover, to standardize paper sizes to an
entirely different 8 x 10½" format. This size was established as the
standard for U.S. Government letterheads, and continued until the
Reagan administration declared in 1980 that the official paper format
for the U.S. government would be the 8½ x 11" size.

At the same time that the Hoover standard of 8 x 10½" paper was
adopted, another committee known as the Committee on the
Simplification of Paper Sizes recommended six completely different
paper sizes. These sizes appear to have been selected merely because
of their being traditional. What later became our familiar letter size
format is simply one of these basic sizes (17 x 22") halved. Our legal
size paper (8½ x 14" / 17 x 28") is also one of the papers specified
by the Committee on the Simplification of Paper Sizes.

As far as can be determined, the 8½ x 11" letter size began to be used
in the United States during or shortly after the First World War.
There was apparently no effort made to prove that this was the optimal
size for commercial letterhead. The purpose simply appeared to be to
reduce the haphazard and chaotic variety of paper stocks and
inventories to the most commonly used sizes.

Paper University: E-newsletter, September/October 2002
http://www.tappi.org/paperu/news/current_issue.htm

Syndicated columnist Cecil Adams, billed as "The World's Smartest
Human Being," offers a good discussion of the history of paper-sizing,
found in the archives of The Straight Dope online:

The Straight Dope: How did 8-1/2x11 and 8-1/2x14 become the standard
paper sizes?
http://www.straightdope.com/classics/a3_016.html

My Google search strategy:

"why is" + "standard paper" +  "size" + "8 1/2 x 11"
://www.google.com/search?hl=en&ie=UTF-8&oe=UTF-8&q=%22why+is%22+standard+paper++size+%228+1%2F2+x+11

I also searched the archives of StraightDope.com, using the keyword
combination "8 1/2 x 11."

Thanks for asking an interesting question. If anything in my answer is
not clear, or if a link does not function, please request
clarification.

Best regards,
pinkfreud

I have you know that I posed this exact same question to the author of
the Imponderables series of books (such as Do penquins have knees? and
When do fish sleep?) - his name is David Feldman and he was stumped!
Great detective work! Thank you!

Dear Contenj,

As Pinkfreud's answer makes clear, the strict answer to 'Why is the
standard size for a sheet of paper 8.5" x 11"?' is that it isn't.  In
almost all of the world, the ISO paper sizes are the standard sizes. 
There is a good explanation of these and their inherent advantages at:

http://www.cl.cam.ac.uk/~mgk25/iso-paper.html

As the author says, "The United States and Canada are today the only
industrialized nations in which the ISO standard paper sizes are not
yet widely used."

Carnegie

...and Canada and USA are the only countries in the world whose 
overseas letter mail causes real problems for every other country in
the world in regards mail processing because of the odd shaped long
and wide envelopes required to hold the odd sized paper used; i.e the
machines are programmed to process local width and lenght envelopes
and those from every other country bar the two in North America. The
North American variety does not go through the sorting machines very
well at all. Similary printers factory programmed to suit US paper
infuriate administrative people in the rest of the world as they ( the
printers) wont recognise the rest of the world's paper used in 95% of
the world's countries, and often have to be reprogrammed  after
continually defaulting back to US specs.  Could you plesae conform to
the rest of the world's ways as a new years resolution. While you are
at it how about throwing out pounds, feet inches yards, ounces, etc
which require difficult multiples of 12s (or 14s in one case) which
now no-one else in the  whole wide world uses having all changed to
the easy 10 X 10 metric system over last 3o years or so.