To take Don's point further.
I tried a test rig version of the game with 5000 total units, ie 2500 each player maximum.
Mostly the game played out fine, though the artillery menu system for humans was completely borked. As were all scenarios that had preplotted arty, mines - basically all save games to include scenarios.
All of that would need a lot of work to recover, and it is not worth the effort.
But as for the engine working with 2500 units per side.
On my machine, the AI would take a minute to pick forces and a couple to merely deploy.
I gave a test of 2500 manually bought Chinese infantry an assault as a test, and Don did that on his lesser capability machine. The purchasing took me abut 2 minutes clicking on its own!. On my machine, it took the AI about 7 minutes to simply do the first turn which was merely a step-off and advance 2 hexes first-turn job. No combat, plotting or firing arty or anything like that. Just shuffle 2500 infantry teams forwards 2 hexes. It took Dons machine about
35 minutes just to do that simple task.
I tried letting an AI force of 65K do an assault all by itself against another AI force, and the arty fires took the first turn to about 15 minutes, with fast arty on. The defenders retreats phase took 4 minutes or so.
As Don said - when a turn had stuff happening, then the save game became about 11Mb.
The replay buffer size was not increased in the test - but if it were, to accommodate 5 times the units, then the save game buffer would likely (WAG) go to the 20Mb plus mark.
If you are playing the AI, then you could simply watch the AI turn for 35+ minutes. But the replay buffer
would be needed for PEM. How many players are really going to sit and watch a 20+ minute replay?. How many PBEM players are actually daft enough to want to push 2500 units about anyway?.
(2500 Chinese rifle sections deployed even on a max size map looked like a barbarian horde of Ghengis Khan days!
.)
Cheers
Andy