。
She raised her voice.
Literal
She [topic-は] voice [object-を] louder made.
声を大きくする means 'to make one's voice louder' — literally 'to make the voice big.' This uses the pattern [noun]を[adjective-く]する for causing a change: 大きくする (make bigger), 小さくする (make smaller). Japanese has many voice-related expressions using 声を: 声を出す (raise one's voice), 声を潜める (lower one's voice), 声を上げる (speak up).