1) In the extended map editor. Size doesn't matter.
2) The battle space is always the same - 10x12, for sieges - 14x14.
3) In AdventureFlybyScene, you need to set MountPoint - the reference point, which approximately corresponds to the location of the attacking hero.
4) The list of dueling arenas is located directly in data.pak. Each arena has its own entry containing the following fields:
CombatScene - a link to AdventureFlybyScene.
ArenaName - the name of the battle arena ("Cave of Corpses", "Last Refuge", "Obsidian Fields", etc.)
Pic - an image depicting the arena, type - Texture. Size - 214x160.
TownType - the type of the town native to this arena.