I want to assume that your fish are juveniles. If yes, they will be around 6-8weeks of age. At this age, 2mm of imported feeds is recommended. As they keep growing and from experience, I have come to conclude with the sizes that correspond to their age especially if they are from good source and are responding well to their feeds. For instance: 2mm at 2months, 3mm at 3months, and 4mm at 4months etc.
However, the type of feed should also be considered, for instance the size of 8mm coppens is the same as the size of 6mm of Indonesia feeds. So put this into consideration and inform your consultant before changing from one size of feed to another.