why cells are small

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!

why are cells small?

Why are cells small indeed? Well, it's really the physical and chemical laws that limit cell size. Substances need to move to and from cells and also within cells. If cells become big, it will take a long time for substances to move from one part of the cell to another and this will not be compatible with life. I have an actual table showing how far substances can move if the distance between the cell membrane and the cytoplasm is in nanometers or in millimeters. However, I cannot find the table right now. I promise I will post it sometime soon. As soon as I find it that is.

Suffice it to say for the moment that one should never expect to find a single cell that is as big as a basketball.