To go into more detail, they always have the same measurement for a certain region of Japan. If you go for 90cm x 180 cm you have a good idea of the measures, that's mostly the space a person (from Japanese average size) needs to sleep.
But if you want to go deep in the details....
Kyoto: 0.955 by 1.91 m (3 ft 1.6 in by 6 ft 3.2 in), called Kyōma (京間) tatami
Nagoya: 0.91 by 1.82 m (3 ft 0 in by 6 ft 0 in), called Chūkyōma (中京間) or Ainoma (合の間, lit. "in-between" size) tatami
Tokyo: 0.88 by 1.76 m (2 ft 11 in by 5 ft 9 in), called Edoma (江戸間) or Kantōma (関東間) tatami
In terms of thickness, 5.5 cm (2.2 in) is average for a Kyōma tatami, while 6.0 cm (2.4 in) is the norm for a Kantōma tatami.[6] A half mat is called a hanjō (半畳), and a mat of three-quarter length, which is used in tea-ceremony rooms (chashitsu), is called daimedatami (大目畳 or 台目畳).
As traditional Japanese length units, a tatami is (allowing for regional variation) 1 ken by 0.5 ken, or equivalently 6 shaku by 3 shaku – formally this is 1.81818 m × 0.90909 m (72 in × 36 in), the size of Nagoya tatami.
In Japan, the size of a room is often measured by the number of tatami mats (-畳, -jō), about 1.653 m2 (17.79 sq ft) for a standard Nagoya-size tatami.
If you visit real state Japanese pages you will see frequently the plans of the houses with the tatamis on them.