I was going to say probably the pineal gland for the smallest, and the cerebral cortex for the largest. The human cerebral cortex is HUGE!
Sure...make me dust off my mother's medical texts, lol.....the largest part is the cerebrum, and the smallest is the brain stem.
If you get really technical, the smallest part could be one of the small glands, like pituitary or pineal gland.
Largest : Cerebrum
Smallest : Midbrain