@schulz
I mean, is there any difference between going with 48px and then zooming in on the map vs going with 54px and then zooming out? Because zoomed-out 54px looks better than zoomed-in 48px?
Politicians answer. My logic goes like this, 2 & 4K screens have lots of pixels and as tech advances they will have more, this means that the standard 48x48px unit gets smaller and smaller on these screens, so now we need to draw the biggest unit icons we can and this is 54px high, width can be lots, lets say up to 96px wide.
Another current hard parameter is 200% zoom.
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How good is the AI in this map?
I will make an outrageous claim and say its the best AI you will play against.
but I would say that wouldn't I.
This map is written for solo play, Black_Elk & I spent ages play testing against the AI.
The forum for GCD has 1.8 million views, so players must think its worth looking at. 
Also I like to think I know how the AI works, if that's possible, see here
https://forums.triplea-game.org/topic/3743/how-to-make-a-map-ai-friendly
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handling of upkeep
Resources are calculated at the being of the players turn, thanks to wc_sumpton. This means losses and gains of TT/SZ and no of units is all done at the start of the players turn.
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handling of terrain & weather
It all appears to work as intended, play it and judge for yourself.
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diplomacy
Has not changed, its all still random, so 1941 GCD does not have any politics, its all scripted. However for FFA maps 1888A has "get impending winners", a set of triggers that forces the AI players to take a step closer to war/fighting the impending winners.