This article is from the Frequently Asked Questions for Linux, the Free/Open Source UNIX-like operating system kernel that runs on many modern computer systems. Maintained by David C. Merrill with numerous contributions by others. (v1.0).
A: Linux needs about 10Mb for a very minimal installation, suitable for trying Linux, and not much else.
You can fit a typical server installation, including the X Window System GUI, into 80Mb. Installing a small Debian GNU/Linux workstation takes from 500Mb to 1GB, including kernel source code, some space for user files, and spool areas.
Installing a commercial distribution that has a desktop GUI environment, commercial word processor, and front-office productivity suite, will claim 15.1 GB of disk space, approximately.
A fully installed Debian GNU/Linux system could use several Gigabytes of disk space.