1. If we compare the lengths, the good answer would be the neurons.
The body of the neuron measures from 5 to 120 μm. The size of a neuron (or rather an axon) varies from a fraction of a micron to two meters (the equivalent from the brain to the toe).
2. The particularity of neurons is that it does not divide, but survive for a very long time.
Neurons do not usually die before the death of the entire organism. However, many types of trauma and pathology can shorten the duration of their existence. Neurons that disappear and are rarely replaced (or not) because there are few stem cells that can differentiate into neurons in the mature nervous system. Like most cells, neurons can die in two different ways: necrosis or apoptosis.