It will be possible with all FreeBSD, NetBSD or OpenBSD.
For FreeBSD it will be
Custom install and select these:
Code:
base [x]
kernels --> GENERIC [x]
man [x]
xorg [x]
then
pkg_add -r fluxbox and you are done.
If you have such small space, then you may create just one big / filesystem.
You may also rebuild FreeBSD base system with custom NO_*= yes options in /etc/src.conf to make it even smaller, but that would require additional 400mb space for /usr/src tree and a place for just built base system in /usr/obj with about additional 200mb.