Question

why the cell is generally of small size

Answer 1

if one cell is damaged by something, then others can take his place

Answer 2

Cells are so little, so they can maximize their ratio of surface area to volume. Smaller cells have a higher ratio which allow more molecules and ions move across the cell membrane per unit of cytoplasmic volume. Cellsare so small because they need to be able to get the nutrients in and the waste out quickly.