Measuring rock fragments

When measuring rock fragments for size class, do you measure across the narrowest axis, the intermediate axis, or the longest/widest axis? This question especially applies to channers and flags that are very thin but also very wide and very long. For example, 1/2 inch (1.25 cm) thick; 16.5 inches (42 cm) long, and 4 inches (10 cm) wide. Is this a channer or a flag?