I was doing some research on an entirely different topic, when I happen to notice something I'd missed before. Here is the official seal of the U.S. Post Office Department, engraved by the Bureau of Engraving & Printing in 1874.
What I had missed was the very tiny image of a postal card - UX1. During the previous year, 1873, the USPOD released America's first postal card, which proved to be very popular. Apparently someone decided it ought to be incorporated into the department's seal. Here is the engraving and an actual card for comparison.
You can see that Charles Burt, who engraved the seal, took pains to capture the details of the card, even including the long line of text although that is so small it is beyond legibility. The rest of the text, though is entirely readable.
So how tiny is this engraving? Well, the actual card is 5" wide. On the USPOD seal, the card is just over 3/8 of an inch! Here a size comparison -- mighty tiny!