Assuming it will be a straight-forward continuation of Metroid Prime 1-3, I'd say 2m is the absolute lowest. We'd get a game that is a combination of excellence and somewhat dated design (plenty of corridors to cover up loading times between the bigger rooms). The Metroid fanbase will be on board with this after roughly 15 years of waiting for a game that is worthy of the Metroid name. (People have to remember that the development of Metroid Prime 4 was rebooted less than two years ago, so a fall 2021 release was basically the most optimistic and still realistic launch timeframe.)
If Retro Studios changes things up, such as pursuing a more open design with bigger seamless areas, then I'd really need to see the game first because assuming a Breath of the Wild-like revelation would be a giant leap of faith in a studio that has failed to live up to its own fame lately (last game released in 2014, so their follow-up project didn't turn out well and wasn't deemed good enough to show off, let alone release).
Regardless, Metroid has inherent sales limitations because it cuts out a female audience and does not appeal to the lowest male denominator. While it wasn't asked by the OP, I'd define the ceiling at 5m, no matter the direction of the game.