It will depend on the USPS collection box, so be sure to read the restrictions on the top of the Blue mail drop box. The traditional blue half circle drop boxes that have a handle to open, will have a sticker on the top that tells you what is acceptable. In general, I would probably try to keep the package size smaller than 4" tall x 12" wide and weighing less than 10oz.
I was able to drop in a small package with pre-printed USPS postage, the package was 3" tall and weighed about 5oz. You might be able to fit a 4" tall package by 12" wide in the standard USPS drop boxes. Make sure any package over 1/2" tall does not use regular stamps. SEE below.
This is what a blue USPS drop box reads in a nearby residential neighborhood:
Mail Collection Policy
Due to federal security regulations and U.S. export laws, DO NOT DEPOSIT the following items in this collection box:
- Any mailpiece over one-half inch think that uses POSTAGES STAMPS
- Any mailpiece that weighs more than 10 oz and uses POSTAGE STAMPS
- Any mailpiece with a customs declaration form that was not completed and submitted online
- Any mailpiece that requires a customs declaration form and uses POSTAGE STAMPS
You must present these items for shipment to a USPS employee at a retail service counter at a Post Office location.
Failure to comply may result in the return of your mailpiece.
Keep the Mail Safe!
Do not mail prohibited dangerous goods or hazardous materials.