na-adjective, noun
magnanimous; broad-minded; grand in outlook
Describes a person whose mind, attitude, or vision is large-hearted and not petty. Literary and somewhat formal; not common in everyday conversation.
彼は気宇広闊な人物で、小さなことにこだわらない。
He is a magnanimous person who doesn't get hung up on small things.
その計画は気宇広闊な構想に基づいている。
The plan is based on a grand vision.
寛大 is more common and means 'generous' or 'tolerant', often in forgiving others. 気宇広闊 focuses on the breadth of one's mind or ideas, not necessarily interpersonal forgiveness.
度量が大きい is a more everyday expression for 'broad-minded' or 'big-hearted'. 気宇広闊 is more literary and emphasizes the grand scale of one's thinking.
A yojijukugo (four-character compound) combining 気宇 (mind/spirit) and 広闊 (broad/wide). The exact historical derivation is uncertain, but it follows a common pattern of classical Chinese-derived expressions describing personal character.