Here is a card size chess computer that you can take with you anywhere to play. The Fidelity Chess Card was introduced in 1990. It is powered by a 0.5Mhz microprocessor and has an estimated rating of 1200 ELO. The Fidelity Chess Card has verify, reverse colors, and other functions.
The Fidelity Chess Card Model 6115 comes with a tiny chessboard with magnetic pieces. You can find out more about it here.