What size is this internal screw for the Macbook Air (A1466)?
The website ifixit.com has repair guides for many Apple computers, including the from-mid-2012 A1266 MacBook Air. 

One of the entries for this MacBook Air is Replace Trackpad. Step 10 contains this picture, showing the screw in question highlighted. Here's a screenshot:



The instruction text says it is a 1.4mm T5 Torx set screw.