Suppose you collect data on the number of scratches found per square meter of finished surface each day. The subgroup size is the number of square meters inspected on a given day. For example, if you inspect 10 parts that are each 5 square meters, and 10 that are each 2 square meters, the subgroup size is as follows:
10 parts x 5 square meters = 50 square meters, plus
10 parts x 2 square meters = 20 square meters, so
total sample size = 50 + 20 = 70 square meters.