Diodorus of Sicily: Greek historian, author of the Library of World History. His activities can be dated between 60 and 30 BCE.
In May 334, Alexander the Great invaded Asia with a large army - larger, for example, than the army which Julius Caesar needed to conquer Gaul. The only detailed account of this army is by Diodorus of Sicily, World History, 17.17.3-4.
The following translation was made by M.M. Austin.
[17.17.3] Alexander then carried out in person a careful review of the army that was following him. Of the infantry there were counted 12,000 Macedonians, 7,000 allies, and 5,000 mercenaries; Parmenion held command over all these.
[17.17.4] They were accompanied by 7,000 Odrysians, Triballians and Illyrians, and 1,000 archers and the "Agrianians". In all the infantry numbered 30,000.note[In fact, Diodorus' numbers add up to 32,000. Among the 12,000 infantery men were the six brigades of Foot companions, i.e., the phalanx, and the three units of Shield Bearers.]
Of the cavalry there were 1,800 Macedonians, under the command of Philotas son of Parmenion, 1,800 Thessalians, commanded by Callas son of Harpalus, a total of 600 of the other Greeks, commanded by Erygius, and 900 Thracian scouts and Paeonians, with Cassander as their commander. The total number of cavalry was 4,500.
Such was the size of the army that crossed into Asia with Alexander.