Final answer:
The barrel size of the iron in thermal waving determines curl tightness, heat intensity, wave pattern, and duration of the style.
Step-by-step explanation:
In thermal waving, the barrel size of the iron determines the curl tightness. A larger barrel size will create looser waves, while a smaller barrel size will create tighter curls. The heat intensity of the iron, on the other hand, is determined by the temperature setting on the iron itself. The wave pattern created by the iron is primarily dependent on the shape and design of the barrel, such as whether it is straight or tapered. Finally, the duration of the style is determined by how long the hair is wrapped around the barrel and how long the heat is applied.