You can put baggage whose overall dimensions are less than 160 cm (the same size as the baggage that can be carried for free in airplane holds) * on the overhead baggage rack of the Shinkansen.

Storage options on the Shinkansen depend strictly on the size of your luggage. For small to medium bags (total dimensions under 160 cm), you can use the overhead racks located above the seats or place them in the generous legroom at your feet. For large suitcases (total dimensions between 160 cm and 250 cm), you are required to make a mandatory reservation on the Tokaido, Sanyo, and Kyushu lines. When you book a "Seat with Oversized Baggage Area," you are assigned a spot in the last row of the car, allowing you to store your bag in the space behind your seat. Alternatively, some newer trains feature "Oversized Baggage Compartments" in the deck area between cars, which also require a specific seat reservation. If you board with an oversized bag without a reservation, you will be charged a 1,000 yen penalty fee per bag. Bags exceeding 250 cm are prohibited entirely; for these, it is recommended to use the "Takkyubin" luggage delivery service, which is highly efficient and common in Japan.