Traditional Japanese Units
Although the metric system is used in Japan, some traditional units of measurement are still in wide use as well.
A common unit of area used when describing room size is jo (畳), which is the size of one tatami mat (around 1.6m²). A tatami is a type of straw mat used as a flooring material in traditional Japanese-style rooms, and has an aspect ratio of 2:1.
A tsubo (坪) is unit measuring an area of 3.3m², the square area covered by two tatami mats side by side. It is commonly used when discussing land pricing. Japanese farmers also use larger traditional units for discussing field sizes.
There is also a traditional measurement system called Shakkan ho (尺貫法), although the use of the old units is not common nowadays, except in such fields as agriculture and carpentry. See the Wikipedia entry for details.
Japanese clothing sizes tend to run smaller than corresponding label units in the U.S., U.K., and Europe, and sizes for larger or taller people may not always be available at average retail stores. For details, refer to the size conversion chart provided by Yamatane Overseas Moving Service.