The entire population of earth could fit into Texas with plenty of room, so fitting the armies of earth into the valley wouldn’t be that much of a problem. After a while it would be even less of a problem.
Where is Armageddon? Armageddon is the Hebrew name forThe mountain of Megiddo. Megiddo is the name of a city which is located in the Plain of Es-draelon, a Y shaped region about 15 miles southeast of present day Haifa, Israel. In the Old Testament it is the Valley of Jezreel, and the Valley of Megiddo. More battles have been fought on this plain than on any other battlefield in the world.
Then they gathered the kings together, to the place that in Hebrew is called Arma-geddon. The seventh angel poured out his bowl [vial] into the air, and out of the temple came a loud voice from the throne, saying, "It is done!" Then there came flashes of lightning, rumblings, peals of thunder and a severe earthquake. No earthquake like it has ever occurred since man has been on earth, so tremendous was the quake. The great city split into three parts, and the cities of the nations collapsed. God re-membered Babylon the Great and gave her the cup filled with the fury of his wrath. Every island fled away and the mountains could not be found. From the sky huge hail-stones of about a hundred pounds each fell upon men. And they cursed God¼ because the plague of hail was so terrible. Revelation 16:16-21 NIV
So either the Bible says what it says or it doesn't. BTW Armageddon is only mentioned ONCE in Rev. 16.
Again I ask - How do you fit all of the armies of the kings into this small area?