Well, I recall reading that we'll have 40 zones in A Realm Reborn (
Source). We'll still have the 5 regions to my knowledge (based on the
official Final Fantasy XIV page), so that'd be about 8 zones per region.
Final Fantasy XI launched with 55 zones (
source), some of which are very small, like Horlais Peak, Altar Room, etc.
That means we'll have 15 less zones (given the "40 zones" thing is accurate, as there's no source cited on the Wikipedia page), we don't know how big they'll be in A Realm Reborn. Going off of the Alpha footage we've seen so far of The Black Shroud, it appears at each part (2 shown so far) are quite large, I'd guess about the same size as Sarutabaruta from FFXI, only with more stuff littered around instead of vast open spaces of nothing.
I have no doubt that the zones in A Realm Reborn will be quite massive with plenty to explore.