If they put Space Bins or Airspace XL bins in their aircraft, then there should be space for approximately one full size carry on per passenger, if passengers put them in properly (they should put signs in the bins showing how to put stuff in them).
Smaller bins that allowed only about 2/3 as many full size carry ons as passengers may have worked when checked bags were included, but not when charging for every checked bag results in more competition for overhead bin space.
Comparison of smaller and larger bins: