Scale
The key decisions for scaling the game are turn length and tile size. Ideally tile size should be such that the slowest unit can move one tile per turn.
Some calculations:
The number of tiles required to have a 1-day timescale in probably prohibitive in terms of processing power, but a turn-length of a week produces a number of tiles that is comparable to existing games.
The number of hours of gameplay to play a Civilisation from 4000BCE to 2000CE is evidently well beyond what players would be likely to invest. However, this emphasises the value of having a random game generator that allows starting the game at any point in history.
