**WARNING -- COMPLETELY UNSUPPORTED SPECULATION AHEAD**
--The Elder Scrolls--
I'll bet you $1000 that it's smaller than Daggerfall* 
But I would imagine each alliances area would be scaled down to roughly the size of Skyrim's map. Would be great if it was at the same scale as Skyrim, but I have extreme doubts as to this reality. So probably in total I would imagine it will be 4x the size of Skyrim for one area for each alliance, and then of course Cyrodil.
--MMORPG's in general--
I'll bet you $1000 that it's smaller than EVE online* 
I would suspect it'll be smaller than WoW at launch, however depending on how many more Xpacs Blizz releases, I hope ESO grows bigger. Also hoep the exploration is more rewarding. Exploration in WoW was annoying in terms of level limits for areas, but fun for some of the silly minigames they made.
However if there are flowers to pick and butterflys to catch, you bet yer ass I'll be exploring the whole gosh darn ES. But I really hope its big as TES has always been big on exploration.
--Thought Food--
You know how Skyrim had the landmass for much of tamriel included? Perhaps those were assets that Bethesda faxxed over to Zenimax, and were used as the base for ESO, which would be exciting because it would mean its the same exact size, which would be extremely exciting, especially as new areas are added. Also excited to check out Vvardenfell whenever they release that. If we keep to Skyrim scale, I know it would be smaller, but that's ok I mean 80% of the friggan map is just ash... (maybe like 60% in pre Dagoth Ur times)
(also hope we get to meet a breathing Sotha Sil
)
*Please not this is not in anyway a legally binding agreement, merely a statement in jest made on an online forum. In no way shape or form should you construe this as an actually wager being placed.