The surface area of one large cell's membrane would be insufficient to meet the cell's metabolic needs. As a cell increases in size, its volume increases more rapidly than its surface area. More chemical reactions occur in general as a cell increases in volume.
More cells allow for better specialization of function.
Insects are composed of cells. Paramecia come from Paramecia. Bacteria are the smallest possible organisms. Minerals are important for good health. Spontaneous generation can not occur.