Hi Markus,

You are free to carry on suitcases without any special reservation as long as they are less than 160 cm in width + lenght + height which is most check-in sized suitcases and bags. It’s quite rare that bags are larger than this.

Myself I’m currently in Japan travelling with a large suitcase, a carry on suitcase and a backpack without reservations on Shinkansen. 😊

Hope it was of help and I wish you a fantastic stay in Japan. Read more about luggage and oversized luggage here:

https://getjrpass.com/oversized-luggage/