As we are talking about .22 rifles here and already mentioned Hensoldt scopes: About 1960 Hensoldt, Wetzlar, made the smallest “rifle scope” I have seen yet, the DIAPI 1 ½ x 7.5. With a length of 137 mm = 5.4” and a tube diameter of merely 14 mm = .553” it is pen sized only.

Here it is mounted on my old Mauser single shot

This tiny precision instrument was designed as a rule breaker, a “non-scope” that is. The then rules of the DSB, the German Target Shooters Union, prohibited the use of scopes in small bore competition. But elderly shooters were allowed to use “optical aiming aids”, provided these did not exceed specified size and magnification limits and featured a simple post reticle. In short order both Hensoldt and competing Nickel, Marburg designed such tiny scopes that qualified as “optical aiming aids” to make use of the gap in the rules. Of course, soon the rules were changed and the gap allowing such scopes closed. So, after just a few years, there was no reason any more to make these miniature scopes. Now, they are quite rare.
The competing, but longer B.Nickel, Marburg, PICCOLO 1 1/2 x.