Japan uses metric system. So, please looking for an apartment or office space in Japan, the size is shown in square meter rather than square foot (in the US). So, if the space is 50 square meters, it is 538.195 square foot.
In addition to sum, Japan uses "TSUBO 坪" to show how large the space is. TSUBO is based on two Tatami Mat size. Two Tatami mats create a square space that is 3.3 square meters. That is 35.52 square feet.
You count each Tatami as one "jo 畳". So if the room has 6 Tatami, it is called 6 jo room. That will be 19.8 square meter or 213.13 square foot.
Now you know how Japanese explain the room size.