Protista
As the name suggests, protists are organisms consisting of only one single cell. Typically a cell as the basic element of organisms is very small. But there are exceptions. The Foraminifera are a group of protists living on or in the sandy bottoms of all seas, reaching the size in the magnitude of cm.
Size of Foraminifera compared to a nudibranch (Phyllidiella sp.) Bali, Indonesia