On a side note, I think it would be pretty cool if other continents besides the known were found. Probably aren't though...and it svcks.
Well to judge whether there are any more large continents, we'd need to know how big Nirn is exactly, and how big Tamriel is in relation to the size of Nirn.
I've heard quite a few things, from it being infinite to being the size of Mars. I don't know enough about lore and get confused, but when I think of Nirn I like to think of it as a planet. When I made my planet nirn globe (http://vality7.googlepages.com/planetnirn), I had a look around for a size, and found this picture of Tamriel on a globe (http://i73.photobucket.com/albums/i201/Vality7/adan_globe.jpg) - which I think was based off coordinates from the Redguard map - and in my opinion that just looks way too small. So I made Tamriel on mine quite a large size in relation to the planet, and while it may look possibly too big, I don't think it is - Have a look at this animation I made (http://www.youtube.com/watch?v=PWiweJS7isA), it shows how much extra ocean there is on the other side of the globe, and even with Tamriel at this size there is tonnes of room for more continents, so I think it's a good size.