Question

what keeps the size of most cells very small? their thin surfaces. their hard shells. their surface area–to–volume ratio. food and wastes.

Answer 1

Their surface area-to-volume ratio.

Cells are so small so the surface area and volume of them can be proportional to each other. This helps with the efficiency of the cell's absorption and waste expulsion processes. Also by the cell's smallness, communication from the nucleus to other organelles is fast and the cell can be regulated while the conditions for diffusion are still ideal.