that in the post Roman times 300 troops could make an army and anything involving more than 50 on each side would make a battle .
But there were also battles of similar size to those in Antiquity in Medieval Europe, so definitely not all engagements were that small.
Another thing is that Medieval sources often mention only the number of knights which fought or died on each side. Sometimes they don't even mention all knights - for example according to Jan Długosz in the battle of Grunwald Polish army lost only
"12 more significant knights killed" (but he doesn't write how many of those not significant died). And knights had their numerous retinues. Apart from knights and their retinues armies consisted also of other components.
Regarding Roman times - the biggest battles compared to mobilizational capabilities were seen during the Punic Wars. The series of defeats inflicted by Hannibal nearly exhausted the Roman Republic's manpower reserves. That was a real bloodbath. After the Punic Wars Rome was also often losing enormous numbers of men in battles - but at that time Rome was already an empire with population numbering dozens of millions, while during the 2nd Punic War territory of the Repubic was limited mostly to Italy. However, already at that time Roman population was bigger than that of Carthage and Roman enemies in near future.
IMHO minor engagements inolving "like 300 or 50 men on each side", as r16 wrote, were also more frequent than large battles in Antiquity. But we simply don't have info about those minor engagements, because of smaller number of surviving sources from Antiquity than from Medieval.
However, large field battles were relatively rare events - just ask yourself how many battles did Alexander fight in Persia or Caesar in Gaul? Skirmishing encounters, hit & run warfare, minor engagements, sieges of small strongholds, sacking of villages & towns - this surely also took place, and was more frequent.
In the Middle Ages situation was similar. The so called "Great War with the Teutonic Order" in years 1409 - 1411, had seen only a few large pitched battles - the most well-known of which are Grunwald and Koronowo. At the same time, there were countless minor clashes and sieges on various scale.
Perhaps the third largest pitched battle of that war (after Grunwald and Koronowo) - was the battle of Bardiów* in 1410 (most likely in November) against forces of Emperor Sigismund - who was an ally of the Teutonic Order. Rather little is known about this battle, but on one Polish history forum I saw estimations, that probably it involved around 5,000 troops on the Polish side and that Emperor's forces (commanded by Stibor of Stiboricz) were more numerous.
It is not sure who was the commander of the Polish army at Bardejov - but most likely Spytek of Tarnów or John of Szczekociny.
*Modern Bardejov in North-Eastern Slovakia.
it doesn't take into account the fact that a modern regiment is about 900 strong while a Roman legion numbered around 5,000, with about as many auxilia again.
Regarding the strength of legion - it depends on period.
Not always it was 5,000 (and not always auxilia was as numerous as legion itself). Moreover, according to what I've read (e.g. John Warry, "Warfare in the Classical World", Adrian Goldsworthy, "The Complete: Roman Army", Gregory Daly, "Cannae, The Experience of Battle in the Second Punic War", Krzysztof Kęciek "Magnezja 190 p.n.e.", Osprey's "Battle of Philippi", etc.) Roman legions were often understrength. The real strength was relatively rarely the same as "paper strength".
Regarding regiments - I suppose that modern regiment is 900 strong on peacetime strength? Or this also applies to mobilized regiments? In WW2 infantry regiments on wartime strength had over 3000 men (authorized / "paper" strength). On peacetime strength active service regiments had perhaps 1500 men or less.
But for example Polish pre-war 56. Inf.Rgt. numbered 1968 men on peacetime strength, due to the fact that its garrison was near the German border. It was regiment of "type II (strengthened)" - regiments with garrisons near the German / Soviet border belonged to this group.
