Super Mario 3D World + Bowser's Fury[/I] game page listing, the file size for the new Switch release seems to clock in at 2.9GB. That includes both offerings: the re-release of the Wii U original, and the "Bowser's Fury" addition, whatever that entails. For reference, the original game was just 1.7GB on Wii U.
Since the reveal yesterday, folks have been speculating as to what "Bowser's Fury" will bring to the table. Is it just a few hours in length? It's entirely possible. It could also be a sort of roguelike side mode that's meant to be replayed, rather than a linear set of levels that's a "one and done" situation.
Any way you slice it, it's not going to take up much room on your Switch. We've reached out to Nintendo to clarify any size listing discrepancies on any other eShop listings just to be sure.
Super Mario 3D World + Bowser’s Fury [Nintendo]