just finished creating my promotional poster/mailers (which I’ll post pics at a later date), but I had to keep in mind that my posters had to legally get through the mail. When designing something to be mailed, keep in mind the standard postage sizes in order to send your work the cheapest way. There are many more stationary sizes than this of course, but these are the most commonly used in the US for basic, commercial stationary systems. All dimensions below are listed as inches.

BUSINESS CARDS
3.5 x 2 (or 2 x 3.5)

SHEETS/CARDS
Letter – 8.5 x 11
Legal 1 – 8.5 x 13
Legal 2 – 8.5 x 14
A7 – 4 x 6 to 5 x 7
A6 – 4.5 x 6.25 to 4.625 x 6.25
A2 – 4.25 x 5.5

ENVELOPES
#10 – 4.125 x 9.5 (for folded Letter and Legals)
#10.5 – 9 x 12 (for flat Letter)
A7 – 5.25 x 7.25 (for 4 x 6 to 5 x 7 letters/cards)
A6 – 4.75 x 6.5 (for 4.5 x 6.25 to 4.625 x 6.25 letters/cards)
A2 – 4.375 x 5.75 (for 4.25 x 5.5 letters/cards)

And check out Printernational.org if you’re curious as to the origins of this system, and/or to delve into the myriad of International stationary systems. It’s cool stuff!