13" vs. 15" is a big difference, and honestly, the difference between a chunky generic 13" model and a nicer compact one (MBA 13" or on the Windows side, various Ultrabooks) is pretty big, too.

Personally, I'd figure out for sure what you have before getting an 11" model. Most people I know manage fine with a 13" ultrabook or MBA in coach, and the 11" -- while even better as a travel-only model for use in coach -- is a bit cramped for general use... and if it turns out that you you have a 15" I'd try the 13" first.

If you current one really is 13", the only intermediate step is a 12.5" subnotebook, and those are closer in size to a 13.0" than an 11.6".

If you're not set on a Mac, the 13" convertible models like the Lenovo Yoga 2 are especially nice for travel use -- they can be used in tent or tablet mode in places too tight to use as a regular notebook, but you still have a 13" screen and a full-size keyboard.