"Sekolah ini luasnya tiga hektar."

Translation: The size of this school is three hectares.

[Sam67858]

“The area of this school...” would be better.

[Rick392366]

Yes, I agree.

"The area of this school is three hectares." = not accepted, reported

[RonLawler1]

area of this school is more precise as hectares are a unit of area - size can mean a number of things but is ok