Why are the cells generally of a small size?

Cells are very small to maximize their surface area to volume ratio. Smaller cells have a better ratio, which helps more molecules and ions move across the cell membrane efficiently for each unit of cytoplasm. Being small also allows cells to take in nutrients and remove waste quickly. 

As a cell grows bigger, its surface area to volume ratio decreases. If a cell becomes too large, it cannot transport enough materials across its membrane to support its volume. That is why cells are always small.