Largest virus: Pithovirus sibericum
The largest known virus was thawed out of 35,000-year-old permafrost and is 1.5 micrometers long. That's too small to see without a microscope, but for a virus — it's huge.
Source: Smithsonian Magazine
Largest animal living now: blue whale. They're also the largest animal we've ever found evidence for.
Source: Christian Science Monitor
Winner, largest living thing: A gigantic spread of Armillaria ostoyae, also known as 'the Humongous Fungus,' which covers 2,385 acres in the Malheur National Forest in Oregon. The fungus's mycelia (the fungal equivalent of roots) spread through soil and tree roots and actually kill the trees it infects.
Source: USDA