Why cells are small — the ratio that sets the limit
A cell's need for exchange scales with its volume, but its ability to exchange scales with surface area. Double the radius and volume rises eightfold while surface area rises only fourfold, so the surface-area-to-volume ratio halves. That geometry, not evolutionary accident, is why most cells sit in the 1-100 micrometre range, and why the exceptions all cheat it: a nerve cell stretches into a thin process, a mammalian erythrocyte flattens into a biconcave disc, an intestinal epithelial cell throws up microvilli that multiply absorptive area many times over. Whenever an exam asks why a structure is folded, villous or flattened, this ratio is the answer being tested.