Biceps brachii flexes the elbow and supinates the forearm, so it will be shortest when the elbow is flexed and the forearm is supinated.
Have your client supine on the table with their elbow flexed and forearm supinated. Place one of the smaller cups over the distal end of the biceps then move the cup up and down the biceps while extending the elbow and pronating the forearm. An easy way to remember this is to think of going from palm up to palm down while extending the elbow.