A lot larger, the frontier is tiny compared to most maps. AC's maps feel big because we are on foot. GTA 4's map which is a relatively small map is 5x the size of the frontier. I'm thinking Watch_Dogs will have a map similar to the size of LA Noire's.
We know that the bridge that you see in the honored trailer is roughly 15 km from the city center. To find mountains near Chicago you need to travel to the mississippi river area. It is however not certain how realistic the map of virtual Chicago is with regard to the real Chicago. Hopefully we know more after the next episode.