Ancient armies could be huge, but each civilization's pecularities had an impact on maximal army size.
For instance the Persians had an gigantic empire which they could mobilize for campaigns, but they never concentrated all of their forces in one place due to the difficulties of logistics. Large scale such as the Persian invasion of Greece under Xerxes required at least a year of preparation to simply gather enough foodstuff for the first few weeks of campaign, the rest to be supplied by sea convoys and from living off the land.
For the Greeks of the classical the land armies were never as huge and they had a lot of troubles feeding the men at Platea, where the armies of many city states coalesced in face of the Persian threat, raising around 100k men to face two or three time their numbers. Usually the city state could only max out their force to 10 to 15k men, but more often much less. I must add that their was an exception to this trend, and that was Sicily as often Syracuse could raise more troops than usual for a polis in order to fight against the Carthaginians, such as at Himera.
In the table 1 pdf file are the troops number given by the ancients for Classical Greece as collected by
J.N. Corvisier, Guerre et société dans les mondes grecs, Paris 1999 pp. 185-188 (Xenophon is the Hellenica. Note that references are mainly to the Bude edition, which may use a somewhat different division than the Loeb used in the English-speaking world). You see that they usualy well under 10000 fighters, even for big efforts such as Mantinea in the Peloponese war, but their is a trend for bigger armies as we go forward in the 4th century and approach the time of Alexander.
The exception to this rule was naval warfare : with around 110 men per ship, a 100 ships fleet meant around 11000 men fighting and we see fleets during the Peloponesian wars that have up to 300 or 400 ships on each side, with once more the fights in Sicily showing larger numbers. For details see the pdf table 2.
To put things in perspective one has to remember the logistical side of things. For instance at the battle of Mantinea Sparta alligns some 7500 men according to Thucydides. If we take the rations the spartan soldiers received during the truce of Sphacterie then this amounts to some 9 tons of barley a day, or the content of some 15 charriots (each could, according to Xenophon, carry some 25 talents, +/- 650kg). At Platea the Greeks forces ate some 90 tons a day during some 12 days. When the Persians captured a large peloponesian supply convoy of either 250 charriots or 500 beasts of burden (depending on which interpretation you follow) they captured between 50 and 150 tons of food, only 1 to 3 days for the peloponesian. At least two or three such convoys were needed every day to supply the Greeks, which they could not organize and explains why they started to retreat before being forced into the battle proper (note that I don't go into the question of the camp followers which may have been with the greek army and could have almost doubled the number of mouths to feed...)
In this context most male citizens are soldiers expected to fight when called upon by the state, something that changes slowly with the rise of the mercenaries that will form an important part of the hellenistic armies. Those armies will also have a core component of professional soldiers and sometime call upon the city levies to bolster their numbers, especially in Greece proper. This explains why they can muster larger armies, but the command difficulties stay the same. Good officers could rarely command more than a few thousands men at once, and it took exceptional ones to lead tens of thousands.
Rome had easier access to manpower due to its peculiar mode of conscripting its former ennemies, thus bolstering her forces. She could thus rather easily raise armies numbering in the 20 to 40k men, and sometime the double. But the largest armies of the Republic (and of the Empire) did indeed proove too unwieldy and often let to great defeats (Cannae, Arausio, ...). Yet they remained a necessity in the east, with forces of up to some 60k men raised for operations against the Parthians (Julian II's expedition). However the nature of warfare in the West meant that such armies were not needed, and the infrastructure to support larger corps disapeared due to pillage, sickness and poverty.
We lack information about warfare between the gothic war of Justinian/Belissarius and the 9th century, but it seems armies were in the hundreds, sometime up to a few thousand men, with the Carolingian having the best logistics (but we don't know for how many men, although the period sees an increase in population, with long abandonned lands being deforested and exploited again). The Carolingian rebirth passed, and the population seems to have fallen once more between the 9th and the 11th century even if large scale efforts are possible, such as Hastings (between 5 and 15k men on each side depending on soures). Then sources come back and show indeed a world were warfare by a few dozen men was considered chronicle worthy, with few larger operations. The late 13th century of the Awans and Waroux in the Hesbaye area is typical of feodal warfare (
https://fr.wikipedia.org/wiki/Guerre_des_Awans_et_des_Waroux) and sees only 300 men on each side. It seems it is mainly in the 14th century that we see much larger scale warfare with the 100 years wars, under the leadership of powerful kings (France, England, ...).
I hope those elements help you answer your question
