It's not really about the size of the books, more about a great world with interesting characters and interesting plots, I guess.
Well, the Earthsea books are pretty special, at least the ones I've read so far. Her prose is amazing, and she deals with much smaller scale problems than most fantasy books. Everything is much more intimate and internal. Characters confront their inner demons or their own weaknesses as much as external problems.
The Dark-Tower books have great characters, a wonderful, gritty setting (and very unique, like King Arthur meets Gunsmoke, meets pop-culture), and they tell a huge story. The first book is very different than the rest, but it is also short.
The Wheel of Time has a great setting, and a great plot. The characters are good, but not as complex as Martin's characters, and the books can drag at times, especially those in the middle part of the series. I still consider it my favorite fantasy series, despite its flaws, because it was the series that made me a fantasy fan. There is a great feeling of adventure, and heroism, and all that stuff, and the books can be really fun.
Also, if you don't mind books aimed at kids and young adults, you might want to think about the Harry Potter series. You've probably already considered it, and may have rejected it out of hand, but they're actually all very solid books--more consistent than most fantasy to be honest. And by the end, the story gets very dark, with plenty of great characters, a suitably epic plot, and a well fleshed out world. The last few books don't really feel like they're aimed at kids, which they probably weren't since the fans grew up as the series was being written. The style of the books changed as the characters aged, and as the readers aged. I think the whole series will go down as one of the best fantasy sagas ever when all is said and done.