An ambitious project indeed
1. I guess you'd have to ask every single C2 developer ever and see what their map sizes are! More realistically, just play around with the layout to see what works best for your project. I've had maps spanning ~20,000 pixels, which may have been overkill for my project. Also, unless you're shooting for one big continuous map, remember you can always have several layouts for different parts of the map to make the game world larger.
2. You can try using Pathfinder behavior to make zombies chase players around obstacles, and Line of Sight behavior to make it possible for the player to hide behind things.
3. Don't know unfortunately. You can try looking through http://opengameart.org/, see what they have.