A listing on Nintendo’s Japanese store shows that just 5.7GB will be required for the digital version.
Other Nintendo-published games are similarly small. ARMS for Switch is just 2.2GB, Splatoon 2 for Switch is 3.1GB, and Super Mario 3D World for Wii U is about 1.7GB.
(=^・ω・^=) Kuroneko S2 - Ore no Imouto - Steam, MyAnimeList and Twitter - PSN: Gustavo_Valim - Switch FC: 6390-8693-0129 (=^・ω・^=)