I like this sort of idea -- playable area on map scaled according to how many players are in flight.
(It has to be number in flight, by the way, because there are so many people who stay logged in while not playing in order to get perks when the map changes. It's not a big effect with player numbers are high, but at 1 am Eastern and later, it can be half or more of the people on line.)
I think it would be good to have 3-4 gradations, not just one at 50 players.
"Winning the war" complicates the implementation of such a scheme, as does the prospect of a pre-chosen area shape not including any of a particular country's bases. So does bombing of HQ/strats. So, exact implementation would take a lot of careful thought, trying it out, and tweaking things based on how it works.
Maybe something like this (where a "disabled" field is akin to the uncapturable fields from the days when capture order was enabled in the game).
-- There are 3 gradations: at 200 players, at 100 players, and at 50 players.
-- A "disabled" base means no aircraft or vehicles are launchable, but guns can be manned, it can't be captured, and all of its associated buildings either become indestructible, or they just repair in 10 seconds.
-- A disabled base is highlighted on the clipboard map.
-- A country's closest current base to HQ/strats is never disabled by the algorithm.
-- At 200 players, a country gets 16 bases, and the rest are disabled. It chooses the ones to keep in order of closest to enemy fields. War is over if the country gets down to 4 bases.
-- At 100 players, a country gets 8 bases. War is over if they get down to 2 bases.
-- At 50 players, a country gets 4 bases. War is over if they get down to 1 base.
Something like that (where of course the exact numbers are subject to selecting better specific values)?