4 in x 1.1 in x 0.13 in / 10.2 cm x 2.8 cm x 0.3 cm
Have someone select any letter of the alphabet; A-Z. By having him simply sort through the 7 wood cards and selecting only those cards that contain the chosen number (turning them upside down so that you have no clues), then you will quickly be able to tell him the chosen letter.
This is an idea from Maurice Kraitchik's 1942 book on titled Mathematical Recreations. There are a couple other related window cards, Amazing Insight 16, and Amazing Insight 99. They can be great fun and are quite astounding in how they work.
On this set there are 7 letter cards and a header card. They are each 4" x 1.3" made from 1/8" thick alder. They are precision made in their Florida shop and the text is all deeply laser engraved.