large-scale work; voluminous work
Emphasizes physical size, length, or scope of a creative or academic work (novel, film, thesis, etc.).
彼の新作は1000ページを超える大作だ。
His new work is a large-scale work exceeding 1000 pages.
monumental work; masterpiece; great work
Focuses on artistic or intellectual greatness, often implying a career-defining achievement.
この映画は監督の代表作であり、日本映画史に残る大作だ。
This film is the director's signature work and a monumental work in Japanese cinema history.
The author released a monumental new work after a ten-year gap.
He spent several years working on this monumental piece.
She finally finished writing that monumental work.
大作 describes a large-budget, ambitious film or work in general, while スペクタクル specifically highlights its visually spectacular, epic quality.