What is the correct pentalobe screwdriver size for a Macbook Air Model A1370?
Based on information from iFixit, you need a P5 Pentalobe Screwdriver, which is 1.2mm, as shown in this MacBook Air 11" Mid 2011 Solid-State Drive Replacement guide to remove the lower case.

You need a T5 Torx driver for the SSD.