Japanese trains (Shinkansen as well) are not adapted to big luggage transportation, as it is not something Japanese people usually do.
Therefore, it is forbidden to bring luggage exceeding 30kg and / or 250 cm (98 inches) total linear dimensions (length + width + height) in the trains.
Since May 2020, booking a seat is required if planning to bring a luggage whose total linear dimensions are between 160 and 250 cm (63 and 98 inches).
However, alternative solutions exist:
Our private guides in Japan can help you choose the appropriate luggage handling as a part of their guiding work.
Note that there are no porters in train stations in Japan.