The game areas will have to be large due to objects (trees, buildings, rocks, etc) will be cluttering up areas. You will be able to use the terrain to your advantage. Since it is an MMO, the maps will have to be large since many players could be on a screen at once with Pokemon battling. However I don't think the maps being large is a bad thing since it gives the feeling of a much bigger world.
That said, there will be limits on how far people will be able to go during battle. For instance we don't want people initiating a battle, then running off to another city and having people stuck. The battle area limit will most likely be the viewing area where the battle was initiated. If we want larger, we will put a faint line where the battle zone ends to let people know not to go past it and give 3-5 seconds warning before terminating the battle.