I need help with molecular sizes?

Water (H2O) is the smallest - it's just one oxygen with two protons attached to it. Potium iodide (KI) exists as separate ions, each with a large hydration shell which increases their effective size in aqueous solutions (including biological fluids) - but they're still single atom particles, so they're smaller than most organic molecules. Glucose is a sugar (a C6 carbohydrate - a fairly small organic molecule), and starches are chains of sugars linked to one another. Membrane pores - depends on what membrane you're talking about. Pore size varies. There are synthetic membranes and there are biological membranes.