Or Window Manager, whichever you prefer.
Which should a minimalist Linux user choose? This has a lot of options, questions, and possible selections.
There is the lightweight window managers such as Fluxbox, IceWM, or JWM (and in some cases, OpenBox). These are rarely over 2 MB in size and are well suited for older computers. For a simple computer, they work well.
Good distros in this area include custom Arch/Debian installations, Absolute Linux, Fluxbuntu (tragically underdeveloped, but it works very well with effort) or AntiX.
Then there are the even more lightweight tiling window managers. They separate the screen into many different tiles that you can swap between, and tend to be very minimalist by default. Terminal knowledge is typically required, though you may be able to get by. These usually come in under 1 MB; sometimes they can fall under 100 KB. Good tiling window managers include Awesome, Ratpoison, or for the more hardcore minimalist, DWM.
The main differences between the various tiling window managers is mostly the feature set. They're all very similar and somewhat related. DWM is at the lowest end of the pole, featuring only a few features to enable (which are all configured at source code build time). Awesome and Ratpoison add more features, such as support for multiple monotors, Freetype support for nicer fonts, and other such fun additions.
There aren't any mainstream distros that use tiling window managers by default, though installing them on a blank slate is fairly easy. Which slate you pick depends on your taste. If the repository version your distro gives you isn't good enough for you, you may need to compile the window manager from source, which isn't nearly as hard as that sounds.
Finally, there are full Desktop Environments. These include Gnome, Xfce, and LXDE. I suggest staying away from KDE; it is very far from minimal and does not try to simplify itself. But, in any case, which you choose is up to you.
Gnome is rather large, but it is simple and can be made minimal very easily.
Gnome distros: Ubuntu, Fedora, Debian, Arch (practically any of them out there)
Xfce is even easier to make minimal. It's slightly lighter than Gnome, but sacrifices a few features and user friendliness for being so.
Xfce: Zenwalk, Wolvix (once it is out of beta), Debian, Fedora's Xfce spin
LXDE is a new DE that is extremely light, but still a bit unfinished. It may take a little bit of work to get running well.