Maintenance for the week of December 16:
• PC/Mac: No maintenance – December 16
• NA megaservers for patch maintenance – December 17, 4:00AM EST (9:00 UTC) - 12:00PM EST (17:00 UTC)
• EU megaservers for patch maintenance – December 17, 9:00 UTC (4:00AM EST) - 17:00 UTC (12:00PM EST)
The data miner who found it says it's the biggest zone in the game, considerably larger than Wrothgar or Stonefalls. However, most of the Red Mountain area appears inaccessible, which would make the overland area similar in size to existing zones. See comparison below:
You can see the island on the current map. If the scale is the same, it looks like the size of Eastmarch and The Rift together; about 65% the size of Cyrodiil.
They literally took the heightmap from TES3 and adjusted it to make the map in ESO, so it'll be nearly exactly the same size. In terms of other zones, it's a bit over twice the size of Orsinium based on some data mining that went on a while back (although that includes the space the Red Mountain takes up, which may not be accessible in-game).
I guess at least 50% bigger than Wrothgar. Vvanderfell as an island is more than twice the size of Wrothgar, but I am not sure that the whole island will be completely opened for exploration.