Let's for now stay close to normality and experiment later. There is no particular limit to size, besides common sense. The planet as a whole is about Earth-sized, so there should be enough space.
And yes, that means the game world is an entire world, spherical and all. And yes, if you want you can detail it down to the street and building level. Tools for all of that will be available in the final game. Yes, that is insane, but I know how to make it happen.