I have a Dell Mini with a tiny SSD, and I use selective sync to only sync my Documents folder. I then symlinked ~/Documents to ~/Dropbox/Documents. It works quite nicely. (and I do sync a few other folders, this is just an example)
As in on device X don't sync folder Y. If you have a photos folder with 100GB of images you can set it so that your MacBook Air does not sync that folder but does get your other Dropbox folders.
