Valorant Install Size

As of January 2021, the install size officially is 12.2GB:

With 142MB download size for the installer:

And 6GB main download:

The total installed file size is 13.3 GB

270M    Riot Client\
13G     VALORANT\

This is likely to increase over time.

According to PrimaGames:

At first, you’ll simply download the install client which is around 65 MB. From there you’ll need to download Riot Vanguard, which is a 3.6 GB anti-cheat system that every player must have before they can play. Once you’ve gotten that squared away, you’ll be able to install the rest of the Valorant files onto your PC, which will bring you to a grand total of somewhere between 7-8 GB.

3.6 GB for an anti-cheat application seems a little steep to me, and other sources indeed mention those 3.6 GB are actually the size of the download, making the 7.3 ("7-8") GB the actual installation size.

GameRevolution, in an article posted around a month later than the others linked to here, mentions slightly higher numbers:

The total download size for Valorant is just over 4.1 GB. However, the game’s file size will more than double after installation. In total, you will need 9 GB of free space on your hard drive to install and play Valorant.

Note that over time updates/patches will likely increase the amount of space necessary.