According to the recommended requirements of Team Fortress 2 on the official Steam store page, Team Fortress 2 will require available storage space of 15 GB. However, the information can sometimes be misleading. It is because, for one, the information is quite old. Secondly, this information does not take into account all the custom content that is added to the game after updates. Once all those things are taken into account, the size of Team Fortress 2 can reach up to 22 GB.
“How does the Steam version of the info say 15 GB and the game actually comes up to 22GB”, you may ask. The thing is, Team Fortress 2 is a very old game. The game is about 14 years old as it launched way back on the 10th of October, 2007. The information that is written on the official Steam store page is for the game when it was launched. At the time of launch, there were no updates for the game which added additional content to the base game. In other words, there were no extra files that were a part of the base game. However, with the passage of time, Team Fortress 2 has changed quite a bit. With the changes, tons of new files have been added to the game, which may increase the size of the base game. Also, if you select a different version of the game with more cosmetic items, you can expect a substantial size increase.