Possibly the largest Eukaryote cells:

These single-celled organisms, called xenophyophores, can grow as large as basketballs. Xenophyophores growing on the sediment can resemble carnations, roses, or lattices, and like corals in shallow water, their bodies create a unique habitat in the deep sea. Though surveys are difficult to conduct at the depths where they live and much of the abyssal plains have not been explored, we do know that xenophyophore meadows may cover large areas and that they inhabit the Atlantic and Pacific oceans. Xenophyophores “represent a little known element of marine biodiversity”. They are also, she added, “very fragile—so vulnerable to human disturbance.

2022-02-24: And the largest Prokaryote:

Thiomargarita magnifica, a bacterium living in Caribbean mangroves is visible to the naked eye, growing up to 2 centimeters—as long as a peanut—and 5000x bigger than many other microbes. What’s more, this giant has a huge genome that’s not free floating inside the cell as in other bacteria, but is instead encased in a membrane, an innovation characteristic of much more complex cells, like those in the human body. It implies the 2 branches of life are not as different as previously thought. The genome was huge, with 11m bases harboring 11k genes. Typically, bacterial genomes average 4m bases and 4k genes. The genome was so big because there are 500k copies of the same stretches of DNA.

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