score:7
I believe one of the most interesting examples is Varosha in Cyprus. It's an abandoned city in the UN buffer zone that was abandoned quite suddenly (I believe people intend to return, so all their things are still there) in 1974 during the Turkish invasion.
In fact there are many examples of UN buffer zones. I don't know whether this is quite "No-mans land" but there are ones on the Israeli borders with both Syria and Lebanon.
Upvote:3
Do the Senkaku/Diaoyutai count? What about the islands in the Kuril island chain that were disputed by Russia and Japan until the last part of the last millenium? None of these are intrinsically large enough to fit your criteria, but the Senkaku's are an example of a small fragment of disputed land that controls thousands of kilometers of ocean rights.
Waht about Isla Aves?
There are a number of disputes on the borders of Saudi Arabia - I think they fall beneath your size requirements, but they seem to fulfill your geopolitical constraints.
Upvote:3
"The Wild Field", or the steppe betweeen Danube to the West, Poland and Russia to the North and Crimea to the South in 16th Cent. The eastern border of that place changed. The area was 0.5-1 million km2. Crimea and christian states were not in prepetual war de jure, but de facto conflicts were so often, that normal life in this area was absolutely impossible.
Even the southern regions of Russia and Poland were almost every year robbed off. The amount of slaves from the North was so huge, that one of the slavetraders seeing crowds of prisoners exclaimed: "Do there remain some people in these lands?". The numbers of slaves taken by Crimean Tartars without any doubt was much greater than the meager million of the african slaves.
There were some safer points in the area that were used as a continious living places. Khortica island on the Dnepr river, for example. But that didn't change the whole situation.
Upvote:4
Historically, the borders between states were not as well-demarcated as they are today, and the regions that comprised the de facto border between two states (hostile or not) are a popular area of study for academic historians.
If you search journals for the words 'frontier' or 'borderlands', you can find many articles on the subject. These areas are of interest because they were an area of contact between peoples, even if the states were technically at war, and it was the location of exchange (linguistic, cultural, religious, economic).
Getting to your question. I think it is almost impossible to answer which of these areas of no definite control was the largest as there was no clear definition of where one state ended and the other began. Imagine the boundary between the Roman and the Sassanid Empires. It was a huge frontier, ever changing over the centuries as the two traded territory.
On an even larger scale, think of the Chinese frontiers to the north and west, and the territory that shifted between them and the Mongols.
Upvote:5
The best current example is Kashmir a large part of which is disputed territory, claimed both by India and Pakistan. My understanding is that the Siachen Glacier would match your description of a no-mans land, where what population existed has been run out by the active attempts at enforcement of the claims across the valley -- however being a primarily a glacier is it hard to believe that many people actively lived there before the conflict.