I often find myself needing to refer back to the USPS guidelines for mailing something, so decided to put the information in a post here so I can find it!. In addition to the maximum and minimum sizes listed above, they want the ratio of the Length to Height of the envelope to be between 1.3 and 2.5 for regular postage. If it is a square envelope it will cost more to mail. Here's the pop-up window that talks about all of this.
There is also a postage calculator on their website to help you know how much mailing will cost for your various projects. You can find that here.