The Postal Service uses the words "flats" to refer to large envelopes, newsletters, and magazines. The words are interchangeable. Whatever you call them, flats must:

The maximum size for a flat provides plenty of room to put lots of material in the envelope. But keep in mind that weight (usually affects rate[1]) – the higher the weight the higher the postage, especially for First-Class Mail.
More about Physical Standards for Discount Flats (PDF)[2] or (HTML)[3].

More about Physical Standards for Discount Flats[4]

Is it possible to fold your flat-size piece to letter-size[5]? If so, you can save money because you'll pay the lower letter rate.

TIP
--What is long? What is high? For flats, "long" (the length) is the side parallel to the address as read. "High" (the height) is the side perpendicular to the length. This general rule does not apply to:
- Automation rate flats.
-Standard Mail Enhanced Carrier Route flats.
There are special size requirements for mailing flats at automation rates[6] and enhanced carrier route rates[7]. Contact your local business mail entry unit for more information[8].

Sizes for cards[9]

Sizes for letters[10]

Sizes for parcels[11]