A Roman Legion was made up of about 5,500 men. The core unit of a legion is the century. Originally a century, as its name suggests, was made up of 100 men. But by the late Republic and early Empire it was actually 80 soldiers and their support staff.
Eighty men make a century. Six centuries make a cohort. Ten cohorts make a legion, with the lead cohort being double-sized. That’s 5,280 men. Add 120 cavalry men and around 100 noncombatants – engineers, cooks, etc. – and you reach 5,500.
Each legion had a golden eagle, the aquila, carried by the aquilifer. They also had a flag with their symbol on it. The flag was called a signum, or a vexillum, and was carried by the vexillifer. Sometimes a legion would detatch a smaller unit. When this happened, the main legion would keep the eagle, while the detatchment marched out under the vexillum. Thus the name for the detatchment became a vexillation.
Legionaries were supposed to be citizens, but by this time recruiting standards were winked at. Many locals were recruited with the promise that if they served Rome well for between sixteen and twenty-five years, they would retire as full Roman citizens.
Some common terms to do with legions:
Legate (legatus) – Either the legion’s commander-in-chief, or else senior commanders under a specific general. For example, Titus is senior legate of the Fifteenth Legion, under the command of his father Vespasian, who oversees several legions. A legate was usually a senator or from a senatorial family, as leading a legion was often a large part of climbing the cursus honorum, the 'path of honour'.
Tribune of the Soliders (tribunus militum) – Not to be confused with Tribune of the Plebs, whose veto power had by this point been absorbed by the Princeps. A military tribune was a staff officer, often in his twenties. The term originates from Rome’s earliest days, when each of Rome’s tribes would send a representative to be a junior officer in the army. Usually 6 tribunes to a legion, the most senior of whom was second in command to the legate.
Centurion (centurio) – Professional, career officer, the backbone of the Roman army. He could be elected, appointed, or promoted from the ranks. Caesar promoted men of valour, and many historians record centurions as being the first over a wall. The most wounded, most decorated, most valuable element in a legion. A general would think nothing of losing all his tribunes, but weep outright if he lost a centurion. 60-66 centurions in any legion (depending on the breakdown of the extra men in the first cohort).
Optio - A centurion’s right-hand, carrying out orders and enforcing discipline. Basically a centurion in training. 60-66 optios to a legion.
Decurian - Cavalry commanders. A legion’s cavalry was divided into four units of 40 horsemen, so 4 decurians to every legion.