Sorry I could not find the table I was talking about last time. However, I found the following notes (which can be converted to a table, but I don't have time, sorry) in my notebook:
In a cell, equilibrium through diffusion is attained within 0.00000456 seconds if the distance from the boundary is 0.1 of a micrometer. If the distance is 1 micrometer, then equilibrium is attained within 0.000456 seconds and if the distance is 10 micrometer, then equilibrium is attained within 0.0456 seconds. If however the distance from the boundary is 1 mm, then equilibrium is attained only after 7.6 minutes while if it is 1 cm, then it would take 12.75 hours for equilibrium to be attained.
If we examine the figures above therefore, it is obvious why cells have to be small, that is, they are in micrometers. A red blood cell for example is about 7 micrometers. Just imagine what will happen if a cell is in centimeters, it would take several hours (half a day, actually!) for substances to move from the cell membrane to the cytoplasm. That is not compatible with life at all! And if the cell is as big as a basketball, can you imagine how long equilibrium by diffusion would take? - months maybe or would it be years? Oh no!