Today I ventured to the deep southwest, to 204.9 204.9, where I found the rim of the world and dropped over it. You walk over some kind of glacier for some time and the there is "nothing" only the himalayas. If you go one step further you drop and drop until you hit something and awake back in Banapur.
I've… seen things you people wouldn't believe… Attack ships on fire off the shoulder of Orion. I watched c-beams glitter in the dark near the Tannhäuser Gate. All those… moments… will be lost in time, like tears… in… rain.
I think the world "bottom" is an ocean with a kill field or somesuch. I seem to remember landing in water on one occasion and then taking damage very rapidly until dead (verse just instant death from the fall), but I'm not sure. Either way the land is built up plenty high from "sea level" it would seem. Guess that was intentional so they could do large complex underground areas if they wanted.
One thing is for sure - Far Cry has a massive world space.