This is the 2423rd most frequent Russian word.
площадью
"Площадью" means "by area" or "in terms of area" in English.
In this sentence, 'площадью' implies the size or space of the apartment given in square meters, serving as a specification.
Моя квартира площадью пятьдесят квадратных метров.
My apartment has an area of fifty square meters.
Here, 'площадью' describes the measurement of area of the city square, providing a detail.
Городская площадь площадью один гектар является местом собраний.
The city square, with an area of one hectare, serves as a meeting place.
In this instance, 'площадью' characterizes the spatial extent of the building housing the museum.