Would someone please explain (or point me towards where it has already been explained) what happens when you choose a speed longer than Normal.
For example, what happens if I choose Epic? More units? More cities? Or are both of those just functions of the map size?
If you choose Epic, then everything goes slower. Research costs more so it will go slower, units and buildings cost more so it will take longer to build them, Great Persons cost more great person points, worker actions take more time and of course you'll have 1.5 times as many turns. Everything is scaled by a factor of 1.5 in epic.
Everything is scaled and that effectively means that there are more decision moments (the turns) but that each decision moment is a little less important. For instance, the decision moments during which you have finished researching a technology are relatively fewer as there are more game turns and an equal number of technologies.
There is one thing that isn't scaled, namely movement. This means that you can move your units more during a game and this has several effects.
- Your units are less likely to become obsolete while moving towards an enemy as research is slowed by unit movement is not.
- You and your enemy cannot (re)build an army as quickly to react to a war declaration.
- Your workers will lose one turn when moving onto terrain which costs 2 movement points and can still move the same distance as during a normal game speed turn, but a turn is worth a little less so effectively your workers are a little bit more efficient.
- As a result, conquest and conquest victories are a little easier.
Since large and huge maps also haven't scaled the movement of units to the larger maps, I would advice to use a slower game speed on a larger map.
Because some things are not scaled (events, the chance of war declaration per turn, the diplomacy requests per turn), I would advice against the speed setting which diverts the most from the standard setting which is marathon. But if you really like the slower game settings, then just pick that one. It isn't that bad, it's just not perfectly scaled.
- if I don't post about this tonight and no one else does - please remind me 
I can't see you losing this game.
