Originally, the game's eShop listing stated that 13.7 GB of free space would be required to download the title to your Switch, but that number has suddenly seen a bit of a bump. The Nintendo store now states that "a microSD card (sold separately) may be required to download this software. Upon release, at least 23 GB of free space will be required".
While the use of "at least" in that sentence has us slightly concerned, it is worth noting that the PS4 version of the game stands at over double that with a 48 GB download required, so it is clear that any necessary refinements and compression techniques to squeeze the game down to a Switch-friendly size have been put in place nicely. It's still a whole chunk larger than Nintendo's first party efforts, however; Zelda: Breath of the Wild was 13.4 GB, Mario Kart 8 Deluxe was 6.75 GB, and Splatoon 2 was only 3.1 GB.
Of course, with the Switch only having 32 GB internal memory - some of which is already taken up by the OS and other necessary files - it is pretty likely that you'll need an SD card to download the game. If you're still in need of an SD card for your console, it might be worth checking out this handy guide on which ones to get.
Are you concerned or surprised about the game's file size? Or is this exactly what you were expecting? Let us know your thoughts down below.