The rules
Up to 160 cm in total travels free. From 161 to 250 cm you need a seat with an oversized baggage area. Over 250 cm is not allowed.
Boarding with an oversized bag without a reservation may lead to a penalty if staff judge your bag too large.
With a Japan Rail Pass, seats with an oversized baggage area are included at no extra cost. Book them by scanning the QR code on your pass at a Shinkansen ticket machine or at any staffed JR desk. Get your Japan Rail Pass
Pass tip: Mizuho and Nozomi
The standard Japan Rail Pass does not cover Nozomi or Mizuho. Sakura and Hikari are covered and have the same luggage options, so book those instead.
Hiroshima day trips
If you visit Miyajima as a day trip, leave big bags in lockers at Hiroshima Station or send them ahead to your next hotel.