Map size isnt really relatable to just simple points values.
It's more about technology - big maps arent good if you are a walking army, even if you give yourself 80 turns. An APC based army has less problems with distance. An army based on Churchill nfantry tanks would have the same problem with vast open plains as the footsloggers.
And especially in MBT, as you progrees through the decades, your basic kit (tanks and APC etc) costs rise noticeably, so a 2000 point 1950 army would be far larger numbers-wise than a 2020 2000 point army. 2000 Points in 1950 may well get you a batallion and in 2020 maybe a leg rifle company team.
I use an 80x80 map for WW2 since my grunts march.
I use the default map size for MBT, and fight about a battalion sized task force.
There is also the factor of weapons reach - modern MBT can engage at distances a WW2 tank cannot dream of. So an 80x80 map might seem to be the choice if your force is walking, but if they are supported by a platoon of MBT, and/or the opponent has them then maybe it would be a bit point blank.
So - "how long is a piece of string" is the question you are really asking here
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