"large army" is correct and usable in written English.
You can use it to describe a large group of armed forces, such as "The King assembled a large army to march to the enemy castle."
It also has a large army.
The Shang dispatched a large army to meet the invaders.
Andrew J. Bacevich: Do we really need a large Army?
No large Army or Marine force was employed.
Another attack came in 653, whereupon Ashurbanipal sent a large army that decisively defeated the Elamites.
Yet, despite careful preparations, the organization of this large army was weak.
The Rwandan Defense Force consists of a large army contingent and small air force.
Albert's large army was present at Mainz when the sentence of deposition was pronounced.
He also insisted on using a large army of climbers who worked methodically as a team.
Alexander imprisoned Pelopidas, and the Thebans had to send a large army to procure his release.
However, conscription raised a large army that turned the tide of the war in France's favor.
