The largest size postcard that can mail at reduced postcard rates is 4.25" x 6". Here is some additional USPS information on the matter.
First Class Mails
The USPS first-class mails are the highest level of service for delivering direct mails like letters, postcards, and packages. If you think of any direct mail campaign sent in envelopes or small packages, it is first-class mail.
Features
It has the most consistent delivery timing; 1 to 4 days.
It has automatic returns and mail forwarding if the recipient is unavailable.
You can track postage with a barcode number
Standard Mails
The standard marketing mail is designed for large commercial mailing campaigns but only when time is not an issue. It is also quite inexpensive.
Features
No guaranteed delivery time.
Mails are typically delivered in two weeks or more.
It does not include forwarding and returning services if the recipient is unavailable.
Both standard and first-class mail are good. However, you must consider the time, parcel size, and postcard mailers cost to determine which one is best for you. So NEVER choose the standard mailing option if you want timely delivery of your postcards.
Depending on the number of direct mails you are sending, our processing time is as follows.
500 - 25,000 1 - 3 business days.
30,000 - 60,000 3 - 5 business days.
70,000 - 100,000 5 - 7 business days.