Final answer:
An endotracheal tube of 2.5 mm internal diameter is recommended for a newborn at an estimated gestational age of 26 weeks and weighing 0.8 kg to accommodate the infant's smaller airways.
Step-by-step explanation:
The size of the endotracheal tube that should be used to intubate a newborn with an estimated gestational age of 26 weeks and an estimated birth weight of 0.8 kg is typically a 2.5 mm internal diameter tube. This size is chosen based on the estimated gestational age and weight, as pre-term infants have underdeveloped anatomical features, and a smaller tube size is necessary to accommodate their smaller airways without causing trauma.