Final answer:
Cells need to be small in size for efficient interaction with their surroundings. As cells grow larger, their surface area-to-volume ratio decreases, making them less efficient. Larger cells have evolved adaptations to enhance cellular transport, but their size is still limited.
Step-by-step explanation:
Small size, in general, is necessary for all cells, whether prokaryotic or eukaryotic. Most cells tend to approximate a sphere in shape. As a cell increases in size, its surface area-to-volume ratio decreases, making it less efficient in interacting with its surroundings.
Larger cells have evolved different structural adaptations to enhance cellular transport, but their size is still limited due to the decline in the relative size of the surface area through which materials must be transported.