The map to the left, facing east, shows the Valley of Armageddon.  The red cross marks Megiddo which overlooks the valley on the west.

The valley that is called "Armageddon" is actually the Valley of Jezreel.  It's a triangular depression between the hills of Lower Galilee to the north and the hills of Samaria to the south.  It's bordered on the east by Mount Tabor, the Hill of Moreh and the Gilboa range.  On the west it's bordered by the Carmel range.  Its long side runs from Jenin to the northern slope of Mount Carmel.  From northwest to southeast it measures about 20 miles and northeast to southwest about 14 miles.  If you include the Harod Valley stretching to the Beth Shean Valley to the east, then you could say it's about 30 miles wide, but I think that's a little stretch.  200 miles long is a big stretch!  At the very most, from Nazareth above the valley in the north to Jenin to the south, it's 35 miles.