The measurement of the distance from the crown of the infant's head to the infant's heel, while the infant is lying on his/her back with legs extended, is a measurement of:

Length.

Explanation:

The measurement may be defined as assigning of the particular number to the measured object. The infants height and weight are measured after the birth.

The measurement of any objects is generally done in terms of the length, weight and circumference as well. The length can be measured in terms of meters, kilometers, feet and inches. The infants head to the heel measurement is done in terms of the length only.

Thus, the answer is length.