The Terratin Incident" **
After being hit by an unidentifiable energy beam the Enterprise crew begin to rapidly shrink.
For myself I think this story is silly (and, yeah, I know the idea was revisited years later on DS9), but in fairness it isn't badly told as the crew is shown reasoning things out. My one question arose after they establish that only organic matter is being reduced...then why isn't the water in the fish tank not shrinking in volume as well? I did find it odd that this rapid reduction was happening so fast that the crew would become quite small within a matter of a few hours.
Shrinking people down to a very small size certainly isn't new in science fiction. The most famous examples are the films The Incredible Shrinking Man
and Fantastic Voyage
as well as television's Land Of The Giants.
I think it was also done on The Twilight Zone.
TAS' effort felt more like Land Of The Giants
rather than The Incredible Shrinking Man
or Fantastic Voyage.
I guess it just doesn't impress me as something that TOS would have even considered doing. I think it goes without saying that from an f/x point this story would also have been impossible for TOS.