Map-size question (And a 'like this idea?' poll)

I've been thinking about a map concept for a while now, and I'm planning to take it into the development stages, but I have a few questions I'd like answered.

Firstly, how big a map is viable for 12-player multiplayer? This is going to be a nature-looking map, so there will be quite a lot of doodads and such. How big can a map be before the lag reaches unplayable levels? There won't be too many units on it, no more than a normal melee, though.
I've been thinking 256x256, if this too big, or maybe I can make it even bigger?

Secondly, I'd like some feedback on the concept itself. At start of game, each player chooses a faction. Avaible ones include:

- Humans. Create a community with villagers and warriors, harvesting natural resources to conquer the world or just maintaining a peaceful village. Depend equally on wood and gold.

- Elves. Basically like the humans, except with different units. Depend almost entirely on wood, harvested through tree-friendly means.

- Ogres. Lead your clan to victory by doing some limited harvesting and relying mainly on pillaging any human or elven settlements. Can harvest what little wood they need by themselves, but are pretty bad at aquiring the gold they need without taking it from someone else.

- Goblins. Create and sell items, technologies and machines to other players, while fighting off anyone who wants to steal it without paying. They have no use for wood, but require massive amounts of gold for new technologies and such.

- Vampires. Hunters of the night, their goal is simply to increase their own power by preying on the weak, and eventually spawn a brood of lesser vampires if they so wish. These undead require no normal resources, but will grow weaker instead of stronger if their thirst for blood is not satisfied.

The game is played somewhat like an open RPG, except only the vampires have a main hero that represents the character. All teams can create hero-type units, and most units will improve with experience, though.

I'm liking the concept. I think it will be a good change in maps... you need to make sure though that every player can't end up picking the same factions... that would be too bizarre.

Map size... I am not a great person for this, but my maps don't seem to lag. I have a map 192x192 for 12 players and it works fine... I also have a decent number of doodads... also if I am not mistaken, trees lag less than other doodads.

I'll probably make separate forces, one for the 'builders' (humans and elves) and one for the others, so people can pick what kind of team they want to play before-hand.

This map seems like a pretty good idea, I was making a similar map like this but abandoned it, because of my horrid terraining skills.

But trust me, 256x256 is WAY too big for a multiplayer map, especially one with many doodads. You should try to keep it around 192x192, and have around 4000-6000 doodads. That's more reasonable for a multiplayer map. Any bigger and the lag becomes so bad that the map just becomes unplayable. Besides, it takes forever to terrain a 256x256 and making it look good, while keeping the doodad count low.

All right, I've decided to go with a 192x192 or 168x168 while playing a bit with unit scaling, to make the world seem bigger than it really is.

Just because you budget for 256x256, doesn't mean you'll need to use it all. Keep in mind that if you're going to need areas off of the main map (caves, inns, castles, anywhere that the players can "go inside" of something), you'll need the space somewhere on the map to put 'em. Don't be ashamed to leave areas blank. If the units can't get there, they can't see that it's empty.