Information MailStream Plus- Where can the USPS rules about size, weight, presort category, preparation, and rate eligibility be found

The main location for all USPS Mailing Regulations and rules is the Domestic Mail Manual (DMM), which can be found at

Just as an example, the physical requirements for Automation Flats are her- 
DMM Section 201 is Physical Standards of mail pieces
6.0 contains Physical Standards for Automation Flats, including weights, sizes and other physical requirements. 
6.1 is the Basic Standards for Automation Flats
6.2 is Additional Criteria for Automation Flats
6.2.1 is the Shape and Size
6.2.0 is the maximum weight.

For *Periodicals Mail* Flats, in addition to the above, there is a section about Automation Flats that can be thicker and heavier than the limits specified in 6.2.  That is DMM section 207.26

Automation Flats are indicated by a Y in the P ATTR parameter position 66.  For Periodical Flats processed under 207.26, an F goes into P ATTR position 66. 

The references given above are for the DMM as-of June 2015.
UPDATED:  December 20, 2017