On FBSD, you mostly would need "minimal install" from sysinstall.
On OBSD, all th stuff with xorg is housed within 528 MB.
Tricks are the /tmp which is used for temporary dstorage of temporary
space for reading compressed files. This can be linked to another place, but I don't do it (still reserve 512MB for them).
On current hard drives, rule of thumb, the *BSD's are happy with 4Gigs once you put the /obj, /*/*/*/*/work files elewhere.
Say, my typical *BSD slices are:
/ 256MB (FBSD, OBSD 128MB)
/usr 3G
/tmp 512M
/var 512M (no www. no postgres, no mail, ... only keeping the system's tree, but linking the effective data elsewhere).
In fact, I still have a /usr/home/*, but share my ~/.icons, ~/.themes, ~/.fonts, ~/Documents, ~/Downloads, ~/public_html, ... everyrhing shareable on a /exports/${user} directory.
Advantage: test an ON (other OS or an OS upgrade/update) at full load.
As I am the guy wearing both a belt and suspenders, I first install an upgradre or OS on a flash stick before testing it it on the hard drive.
Other OSes remain on the flash stick, as Slax, puppylinux or the new OpenSolaris mini cd.
Advantage: can boot on any Ethernet cafe allowing flash booting.
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edit
http://www.milax.org/
the minimal OpenSolaris a la Damn Small Linux, Puppy or Slax.