4" by 9"
One. Go to: http://pe.usps.com/text/Imm/il_007.htm#ep1389181
The different kinds of envelopes available are made for different purposes. The size of the material to be mailed as well as the need for the envelop to be made of more formal or less formal paper are examples of different envelop purposes dictating different envelop design styles.
I wish to envelop you in my arms.
Envelop is a verb -- to wrap up (e.g., envelop oneself in a cozy blanket); envelope (noun) is a wrapper, usually for mailing.
There is no standard size.
First class letter postage from the US to Canada is 0.85 cents in 2012.
There is no standard window size.
What is the standard size of a sofa?
There is no standard size of a factory.
What is the standard-size large envelope?
What the difference between envelop and coherent detection of AM signals
The answer will depend onwhich country you are posting from,where you are posting toin many countries, the size of the envelop.