Of the somewhat commonly owned revolvers the smallest would probably be the Colt Paterson #1, a .28 caliber, 5 shot pistol that could be had with a 1 3/4" long barrel.
It had a folding trigger that would pop out when the gun was cocked and it is estimated that around 500 were made in 1837-1838.
All of the Paterson pistols were rather finicky and their greatest claim to fame is they were Colts first revolvers and Texas liked the large .36 caliber version.
Of the more modern guns I would suggest the Colt 1848 "Baby Dragoon", a 5 shot, .31 caliber revolver that could be had with a 3" barrel.
Called a Baby Dragoon because it looked like a tiny Colt Dragoon pistol it was popular with the gold miners who flocked to California.
It was revised and improved and called a Colt 1849 Pocket revolver, first made in 1850.
The 1849 Model stayed in production until 1873 with an estimated 340,000 produced.
The Remington "New Model Pocket Pistol" was first made in 1865.
It too was a .31 caliber, 5 shot revolver which could be bought with a 3" barrel.
Made in both brass and steel framed versions it looked much like a tiny Remington New Model Army (often called a 1858 Remington).
The Colt 1848, 1849 and Remington New Model Pocket pistols have all been reproduced by various Italian companies.
Although small and underpowered by modern standards, all of these pistols could easily kill a person and were taken seriously by anyone who had one pointed at them for whatever reason.
When one remembers there were no pain killers available and infection could easily complicate a pistol wound it is easy to see why these small guns could be effective.