Question

Why are cells generally of a small size

Answer 1

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. Cells are so small because they need to be able to get the nutrients in and the waste out quickly.

Answer 2

Simply put, bigger mechanisms requires more energy. Smaller cells are very efficient not only because of their ability to work with small reactions, but also because their size conserves large amounts of energy.