。
She is looking for a more spacious apartment.
Literal
[topic-は] more spacious apartment [object-を] is-searching.
マンション in Japanese specifically denotes a higher-end, multi-story concrete apartment building — distinct from アパート, which refers to lower-rent, often older wood-frame multi-unit buildings. Despite the loanword's English origin ('mansion'), the Japanese meaning has narrowed to a particular class of urban housing. 探している is the ongoing form of 探す ('to search for'), describing an in-progress hunt.