In his description of the land snail Carychium exiguum in 18221, Thomas Say indicated that it was the smallest species he had seen. He gave the shell length of his specimens as "more than one-twentieth of an inch". That, of course, is not a very meaningful piece of information, for one can't really tell just how long his specimens were. According to Burch & Jung2, C. exiguum shells are 1.5 - 2.5 mm long.
The left-hand picture above shows the largest land snail species Say described, Neohelix albolabris along with a Carychium exile3 shell. If you look carefully, you will see the Carychium shell sitting on the lip of the Neohelix shell. The right-hand picture is a magnification of the Carychium shell.
1. Say, T. 1822. Journal of the Academy of Natural Sciences of Philadelphia, 2:370-381.
2. Burch, J.B. & Jung, Y., 1988. Land snails of the University of Michigan Biological Station area. Walkerana, vol. 3(#9):1-177
3. I used a Carychium exile shell, because I don't have any C. exiguum. The two species are dimensionally similar enough for the present purpose.