>>5897800MiB+, so 1GiB or more is fine
if you want to fit more shit onto the drive like backups and config files, custom shit, or ventoy with multiple ISOs, get a bigger drive. 8-64 gigs is generally fine if you want to store a bunch of ISOs and some config files on it
if you want to install arch onto your pendrive, it really depends on what exactly you install.
16 gigs for a system with a tiling WM plus a bunch of stuff you might need, not something bloated like GNOME or KDE.
plus what else you install, so 32 gigs is fine if you dont want to install a lot of garbage.