Let's start, shall we?

Marathon. Marathon was the final engagement of an imperial punitive expedition into the Aegean as retaliation for the support of some city-states to the Ionian rebels.
What were the goals? Sack these cities, impose rule friendly to the empire.
Logistical considerations? The operation must be executed through a series of naval landings, greatly limiting the size of thr force – transporting men, horses and equipment overseas was difficult, and, more importantly, very expensive.
Strategic considerations? Judging by the utter defeat of the rebels, and other hoplite forces before them, the western satraps, who were most likely in charge of the operation, saw little threat.
What we know – we are given no number by Herodotus, yet his description of the contingents is revealing – Persians (or maybe the identically outfitted Medes), Saka (Scythians), Ionians and Aeolians.
Estimate based on what we know about the Achaemenids – the basic administrative/strategic military unit used by the Persians was the baivarbam, 10 thousand paper strength. However, due to rotation of men, leaves, sickness, combat casualties, etc., these units would always be a lot smaller than their ideal strength (save for the attendants, whose losses were replaced as soon as possible). Given that there had been a lot of warfare recently, we can make an educated guess in the 7500-9000 range. So that covers the regular infantry. Now come the auxiliaries. Herodotus mentions Scythians – his narrative places them in the center, which is unlikely since Scythians were almost exclusively cavalry. One could assume, that:
1. They did not take part in the battle;
2. They fought dismounted.
There is ample reason to believe they did not actually fight, and to the Greek hoplites all Iranians might have looked the same, wearing the Mede-Scythian dress, scale armor, possibly iron or bronze helmets, carrying spears, swords and axes or bows and swords. Considering this, and the standard proportion of 1:10 cavalry to infantry (as suggested by administrative records), we can assume ~1000 Scythians. The final part, the Anatolian Greeks. We don't really have a clue as to what their size might have been in the ancient sources, yet their intended role is apparent by their position – acting as hinges for the cavalry. A very generous guess could be 5000, although not very likely. So we have an unlikely high total of 15 thousand, so a reasonable estimate could be in the 11-12 thousand range, which means – similar numbers to the Athenian force.

That's for Marathon. I will cover the invasion of Greece some time this or the next week. I suggest you calm down a little after reading this, and try being reasonable. Trust me, there is not much you can do to prove this estimate wrong.

'small Greek force' where?what battle? If I know,I can answer the question…