Funny, I was just working on some of that stuff this week. The maps are measured in pixels, and they range from 800x800 to 3200x3200. Right now, the game recommends a maximum number of players based on map size (e.g. "Recommended Players: 3 or less"). There's really no problem playing a two-player game on the largest map size. Playing a 20 player game on the small map, however, is a problem (mostly due to the fact that the map generator has a hard time packing together that many players on a small map, plus, in terms of gameplay, all players will be almost on top of each-other on the first turn).
There is no fixed number of units in the game. However, you do have to pay a maintenance cost for units (food and oil), so the maximum number is actually determined by how many your nation can support.
Victory conditions, right now, involve being the only player in the game with any cities. (You don't have to capture all the cities on the map, but you have to capture all the enemy cities. In other words: there are neutral cities in the game, and you don't need to capture them to win.) You're also allowed to form teams with other players. If one team has control of all the owned cities, then the team wins the game. I'm thinking of adding a slightly different victory condition involving the capture of 90% of the owned cities. This is to avoid a particular type of stalemate that can occur in some maps.