I think you could run 2.0 + X11 in 4MB in a pinch. I know I ran 2.2 + X11 in 5MB on a cast-off i386SX; tight but useable. If I recall right, 2.0 & 2.2 would run in 2MB without X11 (but a GUI like MGR might fit). 8MB was pretty good and 16MB was positively spacious.

Edit: Add: 2MB/4MB boot with a stripped down kernel, not generic.