Do I need to book oversized baggage space on the Shinkansen?

In general, cabin-size luggage can be brought without reservation. For oversized luggage, an oversized luggage space reservation is required only on the Tokaido, Sanyo, and Kyushu Shinkansen lines.

Refer to the image below for the size limits:

If the total dimensions of your luggage exceed 250 cm, you may not be able to travel with it.