On Aug. 29, Central Japan Railway Company (JR Tokai) announced that reservation of the seat with oversized baggage area in the Shinkansen will be required from May 2020. This measure will ensure a more comfortable train ride.
Nevertheless, the webmaster would rather prefer compartment cars common in Euroean railways. Please see below for more information.
https://global.jr-central.co.jp/en/info/oversized-baggage/index.html