taru-adjective, adverb taking the 'to' particle
Archaic literary word describing vast, open expanses. Modern Japanese normally uses 広々 (ひろびろ) instead.
See also: 広々 (ひろびろ)
曠々たる平原が目の前に広がっていた。
A vast, open plain stretched out before my eyes.
その寺は曠々とした境内を持っていた。
The temple had spacious grounds.
Modern common word for 'spacious' or 'extensive'. こうこう is its archaic literary counterpart.
The reading こうこう is the on'yomi of the kanji 広/曠. The word is a reduplicative form used in classical Japanese.