Legion refers to a large grouping of soldiers. It was generally the equivalent to a corps.
The largest ground force unit in the Breen military was referred to as a legion or hon or hareh in Breen. (LUG RPG: The Dominion War Sourcebook: The Fires of Armageddon)
The Romulan Ground Forces legions were called barhet'val and comprised of between 80,000 to 100,000 soldiers. They were commanded by a high ranking general. (The Dominion War Sourcebook: The Fires of Armageddon)