Aha! .. This question is difficult to explain, but I will try. "Measured" is in the past tense. In this case, you don't use "measured" because if you measure the room now, tomorrow, next week or next month, it will still be 10m across (unless someone has renovated and extended it). That's why you say the room measures 10m across, because it IS 10m across and will remain this way. Hope you understand what I mean.

Because that is the meaning of "measure". It has two usages. I measure the room. The room measures 100m.

Hello Joyce The usage of measures is as a plural noun used to indicate the size and extent of the room. When defining a plural object it would be written as follows: The rooms measure 10 metres across. We would use "is measured" in this way. The room is measured by a qualified person. I hope this helps. Goodbye Rodney