“The Last Drop” by L. Ron Hubbard and co-authored by L. Sprague de Camp
Publication date: November 1941
Original publication: Astonishing Stories
Story description: A special elixir from Borneo that causes people to grow very big or become very small reduces a bartender and a mobster to a combat in miniature.
Currently available as part of the Stories from the Golden Age series, in the title If I Were You.