Look at the 14" laptops made by Dell, Asus or Lenovo. Specs: i5 processor (or i7), 128gb SSD, 4 GB RAM. If you are a modest user, (i.e. not running heavy games or programming) then it will last you for another 5 years. My favorite is the Lenovo T420 which you can get for around $500 used. Tough Thinkpad with superb battery life - they run Thinkpads in the ISS if that's any indication.
Why 14" and not bigger - well it's up to you. 14" is very portable IMO.