Surviving texts mention the denomination 'centenionalis' which must be one of these. Which is not absolutely certain. Finding more texts from that period that mention the matter would be very fortunate. Individual specimens show considerable variation from published norms. Average weights declined over several coin issues which documented that fact with a series of letters in the filelds on one or both sides of the coin.
Study is needed here.
Of course, there are still coin issues that straddle the lines with various specimens being, for example, slightly over or under 17mm. All four sizes were rarely produced together and the system fails to separate the two sizes of AE2 'centenionales'. However imperfect, this system will have a place in describing late Roman coins until all denomination systems are understood. Care should be taken to separate these listings from the millimeter scale used for Greek coins where AE20 is a coin of 20mm diameter. There should be no confusion here since the smallest AE coins are over 4mm diameter.