How big is the slot on a USPS collection (blue) box?

What is the biggest size box that can fit in a usps collection box?

• I'm trying to send off a box to someone and my mail carrier already came early this morning and I missed him and the Post Office is closed so I was wondering was the biggest size box that you can fit in one of the corner blue USPS collection box?

• Answer:

  if you are sending priority or express mail you can request carrier pick up its free so that the carrier will pick up the package now in some cities s/post offices the post office might be closed but the lobby is open they usually have parcel drop slot so you can drop the parcel off