Default install is around 20 GB. If you get rid of unneeded printer drivers, foreign language support, GarageBand loops and iDVD themes, you could save well over 5 GB of space.
If you're really stingy you can get it down a lot further than that...it all depends on what apps you want to run. My old iBook that I only use for Internet and light MS Office work has the install down to about 3.5 GB total.