。
She owns a very big house.
Literal
She [topic-は] very big house [object-を] is-owning.
大きな is the special pre-nominal form of 大きい — only the i-adjectives 大きい/小さい/おかしい have these parallel な-forms (大きな/小さな/おかしな), used exclusively before nouns and giving a slightly more poetic or fixed-phrase feel than the regular form. 所有する is the formal verb for 'to own,' more common in writing and legal/financial contexts than the everyday 持っている.