What is the largest cell in the human body?

The egg cell, about 0.1 millimetres across - just about visible to the naked eye as a speck, and around ten thousand times the volume of a sperm, which is the smallest human cell. That asymmetry is the whole reproductive strategy: one cell carries everything the early embryo will need, the other carries a delivery of DNA and almost nothing else.

A woman is born with all of them she will ever have, and the number falls continuously from before birth - about seven million in the foetus, one to two million at birth, a few hundred thousand at puberty, and a few hundred actually ovulated. Every egg is therefore as old as the woman, which is the underlying reason chromosomal abnormalities rise with maternal age: the cell has been paused in mid-division for decades.

Largest by volume is not the same as longest, which is a different cell entirely - a motor neuron running from the base of the spine to a muscle in the foot is a single cell about a metre long. Both records are held by cells doing something so specialised that the usual constraints on cell size do not apply to them.