From reading the forum it seems many/most? people like to play with large maps. I myself have played 8 games with medium maps, 2 with small.
Is it true the AI plays better in bigger maps? I was always under the impression that the AI was tweaked with medium maps in mind.
The number of opponents also is a factor of course, the more there are, the more crowded it gets regardless of the size of the map.
My experience with medium map,
1 opponent - You get to develop in peace, you can put in place any strategy you want, before engaging the enemy. Less luck depedent than with more opponents. The AI does a pretty good job expanding without any AI opponents to distract it. But It's easy to win if you do a colony rush strategy and win out just by out expanding the opponent so by the time you two meet, you have 75% while he has 25% of the pie.
2-4 opponents - Quite luck dependent. In some games, the AIs war among themselves, leaving me along to carve up a sizeable big amount of turf, leading to an easy win. Other times, I'm engaged in wars early on, while 1 AI quitely develops at the other end of the galaxy and by the time I am master of my part of the galaxy, he has quite a big advantage in terms of land controled.
4+ opponents - Simply wild. You never know what you are going to get. Probably most challenging to play, you have to be highly adaptable.
Colony rush strategies are less effective, because their are more opponents, planets are grabbed up very quickly, so there is little chance of you building an overwhelming advantage that way compared to with lesser opponents. At best you end up with 1 or 2 more colonies than average.