What is the smallest Linux distribution by installation size?

Update: ttylinux is unmaintained at the moment! If you're still interested start here or here.

Depending on your platform, ttylinux is maybe something for you:

This smallest ttylinux system has an 8 MB file system and runs on i486 computers within 28 MB of RAM, but provides a complete command line environment and is ready for Internet access.

Started in 2001 and latest release is from 2015-03-05 so it is still maintained.

The small distro that comes to mind is "Damn Small Linux", with a size of 50 MB (link, wiki)

But also there's even smaller distro "Tiny Core Linux" which is about 15 MB (link, wiki)

OpenWrt is also one of the smallest.