In general parlance, a corvette is a 1000 ton - 2500 ton warship. They are the last rated ships of a navy. Anything below them are not considered rated warships. FACs are generally below corvettes and smaller than 1000 tons.
Of course, these terms have been flexible and smaller vessels are sometimes labelled as corvettes. India has 300 ton corvettes and 3500 ton corvettes!!
But that to me is not a sensible practice, it just creates confusion. Very broadly:
Corvette 1000-2500 tons
Frigate 2500-5000 tons
Destroyer 5000-10000 tons
Cruisers 10000-20000 tons (Zumwalt, Type 55)
Battle-cruisers 20000-30000 tons (there is only 1 left operational, a second one being refurbished its Russian and nuclear propelled)
FAC for 500+ ton boats is increasingly an archaic term, and perhaps the naval sloop-of-war or simply, sloop can be brought back from retirement to describe these boats. While sloops have never been rated warships, they have always held greater dignity than otherwise. "Fast Attack Craft" is just not a dignified designation. Nor does it fit the Azmat class, which, frankly, isn't very fast by contemporary standards. Azmat really is a compromise between a FAC and a corvette. As are some of the Israeli boats, that are also best described as sloops.