I think 10mm is the highest score for the fungal colony to reach or the maximum growth of the fungus( if I'm not wrong) and that is the level you can rate the 10mm diameter length as the higher rate and then rate the other less diameters in low scale ( i. e 10mm = 5, 8mm = 4, 6mm = 3, 4mm = 2, 2mm = 1 and free of disease = 0)

Another scale you can do is by assuming the total area of the fruit is 100% and divide the area into 5 sections if it is free of disease = 0, up to 20% of area infected =1, 40% infected = 2, 60% infected =3, 80% infected =4 and 100% infected 5