I love my 11"... it's great for me. 12" MacBook seems good too, if you don't need ports and want a great screen.
The MBA has become my constant companion, I bring it everywhere. I get around by bike and carry it around in a messenger bag, so I need it to be very light and portable. There are lots of good 13" laptop options, but hardly any in the 11" MacBook Air size with the same power and features. I use it in all the ways someone would a tablet, but I also get to run Logic Pro X. It was a sad day when Apple discontinued the 11".
The durability of the MBA is a known quantity, so that might be worth it compared to the 12" MB. But the 12" doesn't have a fan or really any moving parts so that could be useful too.