Which perm tool determines the size and shape of the new curl configuration?

The perm rod is the correct choice because it plays a crucial role in defining both the size and shape of curls during the perming process. When hair is wrapped around a perm rod, the rod determines how tight or loose the curl will be based on its diameter. A smaller rod will create tighter curls, while a larger rod will yield looser waves. This shaping is essential in the permanent wave procedure, as the way the hair is wrapped around the rod, as well as the size of the rod itself, contributes directly to the final curl pattern after the chemical process is completed.

In contrast, hair rollers are generally used for styling purposes but are not involved in chemical perming techniques. Curling irons create curls temporarily through heat rather than a chemical process, while flat irons are designed to straighten hair and do not contribute to curl formation at all. Thus, the perm rod uniquely provides both the size and shape needed for the new curl configuration during a perm.