This is sort of like asking the question: what the meaning of life?. We have found the USPS is extremely uncertain of it's own written policies.  So let us first discuss what the exact written policy of the United States Post Office is

First-Class letter dimensions:

    At least 3-1/2 inches high X 5 inches long X .007 inch thick
    No more than 6-1/8 inches high X 11-1/2 inches long X 1/4 inch thick
    Maximum weight is 3.5 ounces

    Letters that are considered non machinable are subject to a surcharge.
    Length is the dimension parallel to the address.


 So generally we can conclude that a rectangular piece of mail with it's addressed placed on the horizon that can be run through the automated sorters is first class mail.   So we can see where this is all going as far as determining the correct method of sending a CD mailer via the post office.  something most people do not know is each postal facility makes it's own judgment on mail specs so while one may find no issue another one with a more vague understanding will mark every mailer non-machinable.  

This gets even more confusing when we discuss bulk CD mailers.  Designed to take advantage of USPS bulk mailing rates. We have found the same inconsistency in determining a bulk mailer.  Although UPSPS regulations are very clear on this we have observed the Postal Service charging extra postage because mail pieces are too stiff, too square, or unusually shaped, often jam postal equipment and are difficult to process for the USPS.  When the mail piece jams it usually gets damaged as a result.

We recommend contacting your local USPS  Mailpiece Design Analyst 

 who can help navigate you through the maze of local interpretations of the US Postal code. Don't wait till the last minute!