I chose the 13" as I thought it was the perfect blend of portability and screen size. Though I did give a *lot* of consideration to the 11" and spent many hours over 4 or 5 visits to the Apple store debating the two. The 15", had it been available, wouldn't have interested me for a couple of reasons. First, I already have a 17" MBP, and to me it simply wouldn't have been enough of a reduction in size to warrant having both. Secondly, I really wanted to go small. If I honestly thought I could have gotten away with the 11", I would have gone that route for sure. I spent the 14 days of the return period wondering if I could have gotten away with the 11" instead, and maybe I could have. But I have really come to love this 13" and think overall it was the best choice for me.
Maybe if you don't have another laptop or you are looking for a "desktop replacement," that is lightweight and thin, the 15" may make sense. But I still feel the overall sweet spots for the MBA are in the 11" and 13" models. But that is only my opinion, for whatever that is worth 🙂