Air 11 is the most portable, I was running a 2011 model for 6 years.
Avoid the base model, due to lack of RAM and SSD storage, and you should be fine. I eventually picked up a monitor to dock it at home for doing long writing tasks.
The keyboard on the old model Air is great, they are also surprisingly sturdy. I used mine extensively, passed it on to my mum, who is happily using it for Web browsing and word processing/spreadsheet work.
Buy as late a model as you can afford, I always updated the OS to the latest iteration and never experienced any real issues.