Sure...make me dust off my mother's medical texts, lol.....the largest part is the cerebrum, and the smallest is the brain stem.
Largest : Cerebrum
Smallest : Midbrain
If you get really technical, the smallest part could be one of the small glands, like pituitary or pineal gland.
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!