In many Tokyo train stations you have to be careful with your head, you can destroy router antennas. In order use wireless at train stations and many coffee shops around Tokyo you just have to pay an extra 200-400 yens a month to your home Internet provider.
The size is pretty different. I suppose that one of the main reasons is that Japanese electricity is 110V/50Hz at normal households, so the transformer can be little. At wikipedia there is a very interesting article about electricity around the world with a map were you can check the voltages and frequencies used at each country.