Last we heard about Mortal Kombat 11's physical release was that it required 15.9GB of space for a mandatory download. That info came from dataminers pouring through eShop data, but today we have something a bit more official. According to the game box itself, you'll need to have at least 20GB available for the required download. Again, this download is still needed even if you're going with the retail copy.
It's entirely possible that the download itself needs 20GB, but once everything is installed and set to go, the actual space needed is 15.9GB. Either way, you'll have to start off with at least 20GB free...so start saving for a microSD card!