You can select to either sync all of your iTunes library, or specific playlists to yout iPod or iPhone. You can also specify certain podcasts to sync.
You can also set up "Smart Playlists" and sync only the Smart Playlist. This is great for Podcasts. Or, if you like to revisit those songs you rarely listen to, set it up to sync only those songs with <5 plays, or something.
There's a lot you can do with the Smart Playlists. Check them out in iTunes.
And don't forget that contacts and calendars take up space, too!