Question: What is the size of the buddhi of a child?

Answer: It depends on the degree of its development in the previous incarnations.

Here I should note that the entire mass of the consciousness, which can be very large, not necessarily resides in the body of a newborn child or around it: all the best, Divine, which was accumulated by the soul in the series of its incarnations, is “archived” in a special “container” outside of the body. This “reserve” is called Kundalini.