Outlast 3 is currently in development and there is no exact information about the game's size yet. However, based on the size of the previous two entries in the series, Outlast and Outlast 2, we can estimate that the game will likely require at least 20GB of hard drive space.
Outlast and Outlast 2 were both around 15GB in size. Outlast was initially released on the PC in 2013 and was sized at around 14GB, while Outlast 2 was released in 2017 and was about 16GB in size. So, if Outlast 3 follows the same pattern, it can be expected to be around 20GB or more.
On the other hand, the game's system requirements are likely to be more demanding than those of its predecessors. Outlast 2, for example, requires at least 8 GB of RAM and a minimum of 4 GB of dedicated video RAM. This means that Outlast 3 may require more RAM and a more powerful graphics card that would take up more space.
In conclusion, it is difficult to predict the exact size of Outlast 3 until the game is released. However, based on the size of the previous two games in the series, we can expect the game to require at least 20GB of hard drive space.