The largest cell found in the human body is called a red blood cell. These cells are made of plasma and hemoglobin, which carry oxygen throughout the bloodstream. Red blood cells have no nucleus or organelles, and their sole function is to carry oxygen from your lungs to other parts of your body. They do this through a process known as diffusion, where they release molecules into tissues that need it while taking up carbon dioxide from those tissues, which can be released back into the air we breathe out when exhaling. This large size allows them to easily circulate around our bodies; however, these cells only survive for about 120 days before dying off and being replaced by new ones.
What we think of as a cell is actually made up of the following: the plasma membrane (cell wall), cytosol (water and salts), chromatin (DNA and proteins), the nucleolus (RNA and proteins), various organelles such as mitochondria, endoplasmic reticulum, etc., and an outer membrane. Red blood cells have no nuclei or organelles, making them the largest cells in the human body at 9 micrometers. The smallest cells are around 0.1-0.2 micrometers, which is less than a tenth of the size of a red blood cell.
While other types of cells such as neurons can be much larger than a red blood cell, they are still individual cells, so the largest of all cells in the human body is a red blood cell.