I flip flop using windowmaker and fluxbox (which is similar to blackbox) and they are much faster on my machine than kde or gnome.
The best site I could find for actual size is this:
but some of those I've never tried (or heard of for that matter).
Blackbox looks like the smallest one I've used at 230K. Fluxbox would probably be a similar size
Windowmaker clocks in at 1.8M
If you want to look at fluxbox take a look at this site