Well, if my calculations are correct...
We have Horizon's map in the middle. Pink is that very map x3 (So, more or less the estimated size of Horizon 2 map) and in green, we have Oahu, from TDU.
We have to remind something. Horizon map is much smaller than that, since in Horizon a lot of the terrain is not accesible at all, so, if we only count drivable space, Horizon 2 map will be smaller.
The good thing about TDU was the long travels, but most of the map was...the same and the same. I prefer a smaller map, but with more things to do...