Nummular is a term to describe a lesion size. Nummular size is about a coin size, or 3cm x 5cm. Other sizes could be miliar which was about a dot, lenticular which was around a lentil, and plaque which sizes around a palm of the hand. This size description is not really descriptive since it has various ranges. Actual size in centimeter unit might be better to describe a lesion.
