en.wikipedia.org/wiki/Rai_(unit)
It correctly says that the Wah (or Talang Wah) is exactly 2m. The Rai is 400 square Wah or 1600 square meters.
The site currently states 1.91m for a Wah (with an indication that this value is an approximation).
en.wikipedia.org/wiki/Rai_(unit)