Question

Why cells are so small.

Answer 1

Cells are generally so small in size because of two main reasons.

1. Different regions of a cell can communicate with each other rapidly for the cell to function effectively.
2. Cells need to have a large surface area / volume ratio for greater diffusion of substances in and of the cell.

If the gets larger then the surface area to the volume ratio gets smaller and beyond a certain limit, not enough material will be able to cross the membrane fast enough to accommodate the increased cellular volume.