The Kali Linux installation guide says it requires 10 GB. If you install every Kali Linux package, it would take an extra 15 GB. It looks like 25 GB is a reasonable amount for the system, plus a bit for personal files, so you might go for 30 or 40 GB.

How much GB does Kali Linux need?

System Requirements

On the low end, you can set up Kali Linux as a basic Secure Shell (SSH) server with no desktop, using as little as 128 MB of RAM (512 MB recommended) and 2 GB of disk space.

Is 8GB enough for Kali Linux?

1 GB of RAM and 20 GB of hard disk is the minimum requirements for kali linux. But if you want to run it smoothly get atleast 2 GB of RAM. I run it on 8GB RAM that too dual booted with Arch Linux. Install it and feel the power of hacking.