For "a few inches" read "under two inches in all but the very largest examples".
For "5-10mm dia" read 3-5mm dia in all but the very largest examples.
For "hardened steel" read "mild steel in all but the very few and rather expensive examples".
All of this information is easily discoverable with a few seconds of research, yet the myths and postulations prevail <sigh>
PDR
OK, the steel shaft in the DJI phantom 2213 motor is just over 36mm long and 7.9mm diameter which is less than 2 inches but more than 5mm thick.
DJI ESC and Brushless Motor
I can't speak for the DJI motor shafts, but all the aftermarket motors such as scorpions, Emax advertise that the shafts are hardened. I would be surprised frankly if the DJI shafts are not, as it's not a hugely expensive thing to do and has lots of advantages.
S-5525 Shaft Kit - Scorpion Power System
http://www.merqc.com/files/Datasheet/emax.pdf
There will also be an ounce or so of mild steel in the stator. If you know of any certification standards that suggest a jet engine should be able to ingest four of these without becoming very unhappy indeed - or even some smaller steel items - then please post links to them.