I need a GNU/Linux distribution that is:
Small: Less than 500 MB (after installation) for the "base system" with coreutils and a window manager, ideally without having to uninstall a bunch of pre-installed software first.
Fully free/libre: It must not automatically install proprietary software. That also means that it can’t use an unmodified Linux kernel (as it contains proprietary binary blobs), so it should use the Linux-libre kernel or something similar.
Actively maintained: Security updates should be made available reasonably quickly for all supported packages.