As far as I know, New World is located West of Eorzea and Meracydia is south of of it. AS far as size, I'm not too sure, I assumed that Meracydia would be about if not bigger than Thavnair.
I have been looking forward to going to these places for years now but even though the map from Shalrayan to the far east has been fully labeled if you dig deep enough (not in-game).
The problem you see is that I haven't come across any map that displays the size, geography of either New World and Meracydia. Perhaps it's viewable from the moon maybe? Does anyone know yet? If so, could you care to share a picture of said maps even if it's a rough drawing?
^ Meracydia probably (screencap from 6 years ago)
Known map, this was made about 6 years ago as well