Below are downloadable PDFs of common sizes of pieces that you might need to mail. Complete with off limits areas, designated return address and indicia areas and sizes to accommodate bleeds.
4.25 x 5.5 postcard
4 x 6 postcard
5.5 x 8.5 postcard
5 x 7 postcard
6 x 9 postcard
6 x 11 postcard
8.5 x 11 half fold mailer
8.5 x 11 tri-fold mailer
8.5 x 14 half fold mailer
8.5 x 14 tri-fold mailer
11 x 17 half fold mailer
11 x 17 half-then-half mailer
11 x 17 tri-fold mailer
Helpful Resources
Postal Explorer
http://pe.usps.gov/
Ribbs
http://www.ribbs.usps.gov/index.html
Zip4 lookup
http://zip4.usps.com/zip4/welcome.jsp
