This article is from the misc.writing Writing FAQ, by Wendy Chatley Green email@example.com with numerous contributions by others.
A manuscript is not an opportunity to show off your
elaborate desktop publishing system. Many publishers
specify a font or type size in their writer's guidelines and
only a fool would ignore such a requirement. Editors read
vast numbers of pages and anything that strains their
eyesight gets a deserved toss toward the reject pile.
If no font is suggested then the writer should
choose one that does not distract from the writing. Serif
fonts, which have slight projections to finish off the
stroke in each letter, have been proven to be easier on the
eyes than san-serif fonts, which resemble block printing.
Whether the font is proportional or monospaced also
affects how easy it is to read. With proportional fonts
such as Times, the individual characters vary in width ("w"
is wider than "i.") With monospaced fonts such as Courier,
all characters are the same width. Proportional fonts make a
manuscript look more like a book and allow more words per
page but monospaced fonts give editors a more accurate feel
for the space required by the piece.
Size also matters, at least for fonts. The usual
size is "12 point" (also referred to as "10 pitch" or
"pica.") Pitch refers to the number of characters per inch.
Point size refers to the relative height of the font; a
point is a typographical measurement very close to 1/72nd of
an inch. Anything smaller than 12 point or 10 pitch and
editors might strain to read the words; use anything bigger
and editors may assume that you are disguising a too-short
Although any legible font might be acceptable, the
safest choice is Courier 12. Work printed in Courier 12
closely resembles typewritten work. Familiarity with
Courier allows editors to quickly extract word count and
other important information from manuscripts printed in it.