Slax is a linux which is built by the user. Additional software can be added and removed, using Slackware packages and Slax modules. A traditional package manager such as Debian's APT is not required to load additional software.
Slax modules are completely self-contained. Users can also modify the default CD image or USB drive installation to customize the packages available in the distribution on boot. Slax also allows Slackware packages to be converted to Slax modules with the tgz2lzm command.
Like if you want to build a Slax linux which is used for wireless hacking you can make one includig all hacking tools like - Aircrack, Airsnort, madiwifi, wireshart, Kismet etc. Go here to know more.
This makes the whole size of the OS very small as only those modules are included that the user actually wants. The best part os the linux is that you can always add more to it.
A basic Slax linux image is just 200 mb. There are many already customised version available amde by various communities.
You can also go online and add modules to your selection and when done download the OS image. Isnt that just great. So go and create a linux that suits your needs coz 200MB isnt much for an amazing linux.
Download slax here- http://www.slax.org/