Under or over 160 cm
Many 28 inch cases total 156 to 165 cm. If yours is 160 cm or less, it travels free. If it is 161 cm or more, you need a seat with an oversized baggage area.
If you are not sure
When in doubt, reserve the oversized baggage seat. With a Japan Rail Pass it costs nothing extra, and you avoid any discussion with staff.
Travelling with a Japan Rail Pass? Seats with an oversized baggage area are included at no extra cost. Get your Japan Rail Pass