Tatami (which are actually about 1.5 square meters in size) are made of high quality rice straw that forms a relativey smooth surface. They are very thick, a little over 5 cm. The reason why they are the traditional floor cover in Japanese homes is that they are relatively soft, making them very comfortable to kneel on, and kneeling is traditionally prefered to sitting in Japan (it actually is better for the spine). Normal room sizes in Japanese homes are 4.5, 6 or 8 tatami.