1.6 Alternator Pulley

Does anyone know the correct diameter of the alternator pulley. My car came with three alternators. It was fitted with a 40 amp Motorola with a 70 mm pulley. This was obviously incorrect and the drive belt was very tight with this pulley. Among the spares was a Ducellier type 516002A with a 70 mm pulley and a 516002B with a 50 mm pulley. I fitted the latter because instinct told me it's better to use the smaller one because the alternator would be driven faster and the belt fitted properly. It's working ok but I'd like to know whether I'm using the wrong pulley or the wrong drive belt.