How much data is used during factory reset recovery mode for the Mac OS?
Each size (Internet Recovery Mode and OS X reinstall) is comparable to the sizes of the respective dmg files containing the Base OS X/full OS X install.

So booting once to Internet Recovery Mode requires the download of about 650 MB and restoring your original operating system – which depends on your MacBook Air model – requires an additional download of about ~4.7 GB (Lion) to ~5.7 GB (Yosemite).

You can't download and install El Capitan directly because the original system of the newest MacBook Air was 10.10.2.