22, unless it's 23. But sometimes it's 23.35, but technically 23.15.
Then there's this, if the threads are 23 x 0.0.
Next question?
/s
-Kurt
P.S.: Jokes aside, nothing special about Campag square taper. 22mm. You may find that you'll need a very thin-wall 15mm socket to pull the crank bolt though.