Today, the smallest postcard or envelope that you can mail through the US Postal Service has to measure at least 5 inches long and 3-1/2 inches high. For comparison, a #10 business size envelope is 9-1/2 inches long and 4-1/8 inches tall. There was not always a minimum size for mail items, however, and these three examples prove that point. These are entire envelopes, not cutouts from larger pieces.
Figure 1 is a cover from the early 1860s that fails on both dimensions. It is just over 4-1/2 inches long and not quite 2-3/4 inches high. It is franked with a common 19th century US stamp that is #65 in the US Scott Catalog. That stamp was first issued in 1861.
The town in the postmark is not well struck, but it is very likely Bethel, Connecticut. The cancellation is from November 15 or 16, and while no year appears in the postmark, there is a good probability it was 1861. I base both those assertions - the town of origination and the year - on the addressee.
The letter is addressed to "Mrs. David A. Beebe/Care of Havemeyer, Townsend and Co." Havemeyer, Townsend and Co. was a sugar refinery formed in 1855 and had an office at 102 Wall Street in New York. David Beebe, who lived in Brooklyn, worked for that firm. David and Mrs. Beebe - Julia Rebecca Benedict - were both born and died in Bethel, Connecticut. Thus, this letter was likely from a friend or family member in Bethel to Julia.
I base the year on the fact that in late 1861 or early 1862, Havemeyer, Townsend & Co. was renamed Havemeyer & Elder after the death of one of the founders in November 1861 and the retirement of another sometime in 1861, Of course, it is possible that the sender was unaware of the name change by November 1862 (or even 1863), but that seems unlikely if the sender was acquainted with the Beebe's.
The second cover in Figure 2 would meet today's 5-inch length requirement but would fail the height requirement by well over an inch. Like the cover in Figure 1, it is franked with a 3-cent US Scott #65 and was postmarked in Utica, New York on what looks like December 1. The year is very hard to make out, but it was probably 1861 or 1862 because the recipient of the mini-letter, Maria Hill Merriam, was married sometime in 1863 to William Franklin Brush and gave birth to her daughter Adeliza Frances Brush on December 14, 1863.
The last mini-letter in this set, Figure 3, would have just met today's length requirement but like the letter to Ms. Merriam, its height was more than an inch too short.
The cover was postmarked January 31 in Glendale, Ohio, but I have no idea in what year. The stamp even defies identification. The design of this 3-cent stamp first appeared in 1870, and the first version of it had a "grill" - a set of raised points designed to prevent cancellation marks from being removed from the stamp. This stamp shows no signs of a grill so that eliminates one possibility. Unfortunately, this stamp is printed well off center, and that prevents me from determining whether this stamp has a so-called "secret mark" or a marker that would show it was printed from a re-engraved plate. The best I can do is to place this cover sometime between 1870 (when this design first appeared) and 1883 (when the rate for first-class mail was reduced from 3-cents to 2-cents).
The recipient of the tiny letter in Figure 3 was Kate Ehricke, a music teacher in Albany, New York. From available census records and Albany city directories, Ms. Ehricke resided at 69 Hamilton Street in Albany from at least 1874 to at least 1880, which also doesn't help narrow the year this letter was sent.
Figure 1 - Envelope measures 4.65 in. by 2.64 in. (11.8 cm. by 6.7 cm.)
Figure 2 - Envelope measures 5.27 in. by 2.17 in. (13.4 cm. by 5.5 cm.)
Figure 3 - Envelope measures 5.08 in. by 2.36 in. (12.9 cm. by 6.0 cm.)