At a bare minimum, if you don't want the system sources, I think you can
probably get by with around 240MB plus swap space and temp space.  (My root
partition is using 67MB, and /usr (after discounting X11, sources, and
/usr/pkg) uses around 171MB.)  The useful /usr/pkg items could bloat that by
2-300 MB, depending on your tastes.

I know that I ran NetBSD very comfortably for a long time on a 699MB drive.
Certainly any drive that's even remotely new is likely to be plenty large
enough for at least a spartan text editing system.