。
She owns a large estate.
Literal
She [topic-は] big mansion [object-を] is-having.
屋敷 ('mansion, estate, large residence') is a notch up from 家 ('house') — implies grandeur, large grounds, often historical or aristocratic associations. 'Mansion' or 'estate' are reasonable English equivalents (note that the loanword マンション in Japanese has the unrelated meaning 'condo / apartment building,' a famous 'false friend'). 持っている for possession of physical property is standard.