Justin (generally referred to as Justinus Frontinus) is significant historically because he wrote an important epitome of a universal history. Justin abridged the writings of the first century B.C. Roman Trogus Pompeius. Trogus had written a magnum opus in 44 books -- most of which are now lost. It's known as the Historiae Philippicae. In the following passage, Justin provides one set of figures (guestimates and presumably those of Trogus Pomepius) of the forces of Xerxes. The source of the passage is a public domain translation of Justin excerpted from Marcus Junianus Justinus, Cornelius Nepos, Eutropius, translated by Rev. John Selby Watson, M.A. (H. G. Bohn, 1853).
"II. X. Xerxes then proceeded, during five years, with his preparations for the war against Greece, which his father had commenced. As soon as Demaratus, king of the Lacedaemonians, who was then an exile at the court of Xerxes, understood his intentions, he, feeling more regard for his country, notwithstanding his banishment, than for the king in return for his favours, sent full intelligence of the matter to the magistrates of the Lacedaemonians, that they might not be surprised by an unexpected attack; writing the account on wooden tablets, and hiding the writing with wax spread over it; taking care, however, not merely that writing without a cover might not give proof against him, but that too fresh wax might not betray the contrivance. These tablets he committed to a trusty slave, who was ordered to deliver them into the hands of the authorities at Sparta. When they were received, the object of them was long a matter of inquiry, because the magistrates could see nothing written on them, and yet could not imagine that they were sent to no purpose; and they thought the matter must be momentous in proportion to its mysteriousness. While the men were still engaged in conjecture, the sister of king Leonidas surmised the writer's intention. The wax being accordingly scraped off, the account of the warlike preparations appeared. Xerxes had already armed seven hundred thousand men of his own kingdom, and three hundred thousand of his auxiliaries; so that there was some ground for the assertion that rivers were drunk up by his army, and that all Greece could scarcely contain it. He is also said to have had a fleet of twelve hundred ships. But for this vast army a general was wanting; for if you contemplate its king, you could not commend his capacity as a leader, however you might extol his wealth, of which there was such abundance in his realm, that, while rivers were drained by his forces, his treasury was still unexhausted. He was always seen foremost in flight, and hindmost in battle; he was a coward in danger, and when danger was away, a boaster; and, in fine, before he made trial of war, elated with confidence in his strength (as if he had been lord of nature itself), he levelled mountains, filled up valleys, covered some seas with bridges, and contracted others, for the convenience of navigation, into shorter channels."
While such estimates, however unreliable, have value for those calculating the relative forces of the two sides, there is more of interest in this, the Persian War section of Justin, as you may have noticed. Justin provides an anecdote about one of the exceptional Spartan women, the intelligent sister of the Spartan king Leonidas who understood that the wax of the writing tablet covered something of interest.
In the 12th section of Book II, two after this passage, Justin writes about another exceptional woman, Artemisia:
"The king, meantime, remained on shore as a spectator of the combat, with part of the ships near him; while Artemisia, queen of Halicarnassus, who had come to the assistance of Xerxes, was fighting with the greatest gallantry among the foremost leaders; so that you might have seen womanish fear in a man [Xerxes himself], and manly boldness in a woman."
