Then if it's not possible to find out the size, I want to ask this question.
Is 100 sq m enough for a news studio?
Depends what you want to do.
The studio that the Newsnight (and the Six O'Clock News) set is in is approx 3500 square feet, or approx 325 square metres. The actual set is a lot smaller than this, and is larger than needed for a straightforward big for a news show.
100 square metres should be fine for a relatively straight forward news programme - it's not huge, but not impossibly tiny either.
Of course other issues other than floor area, like ceiling height can't be ignored - as low ceilings cause massive lighting compromises. (The lower the ceiling, the more difficult to keep key lights off backings)