Small Tetris by Tore Bastiansen sure lives up to its name. Its executable is below 2kB, still a bit smaller than Ben Hanke's TinyTris (which, however, has graphics), less than half the size of Marko Kleine Berkenbusch's Tätris, and several times smaller than the other contenders, TetraFix, Tetrix, and Stakan. Small Tetris uses 40 column color text mode, it should be played with a color monitor only. If you are playing it under Windows in a window, set it to an 8×8 font, which will give you square albeit tiny blocks.