I'd say also full sized. My reading of it is:
Saga: Full sized campaign using existing engine iteration focused on a tighter area/timescale
Next Major Historical: Full sized campaign using new engine/new engine iteration, focused on a broader area and timescale/era.
The major difference isn't the size in gameplay terms of the map, but the size in real world terms of the area covered, the amount of real world time covered, and what's being used "under the hood" as it were.
At least that's my understanding anyway.
All the Best,
Welsh Dragon.