Exact final map sizes and such are subordinate to gameplay and technology.
For example, maps should not be too crowded, nor too sparse with players, but we want more players on them than any other melee game can muster.
Testing will be the final reveal to what will become the "right" mapsize for the "right" amount of players, as this is very hard to quantify otherwise. We want to push it as far as reasonable hardware and good netcode can do.
But the goal is.. as you know: Epic!