If we go with the high end of that estimate and assume 10 big islands, plus my assumption that the big island is 80x80WUs, that's 800 WU space from the islands to the edge of the map.
Forge World's space, ignoring the fact that a lot of it is not Forgeable:
Forge Island's space, with the assumption that big island is 80WUx80WU... and I deleted 200 WU from each side to make it a more conservative guess. It should be noted that coordinates in Reach and Halo 4 cannot go past 1000 or -1000 or else it'll wrap around when you try to edit it, so you can't place anything past 1000 WUs anyway from the center of the map
So basically, they just said "fuck it" and went to the limits of the engine (or file format, whichever one sounds better to you)
tl;dr GOD DAMN THAT'S A LOT OF SPACE