Unanswered questions of the month

Why are the (human) cells so small?

I really entered a crisis here. I tried to improvise an evolution-based explanation, involving the transition from procaryotes to eucaryotes (which does not quite answer why the procaryotes are small, anyway), but it did not sound convincing. Then I lamely tried to answer back with another question, asking why are humans the size they are, hinting a shy parallel. I was not any convincing. Another attempt was to suggest Lorenzo that if things have worked out well this way, mother nature has thought of not trying to change it, otherwise it may have risked to change for the worse. That our single cells are small enough not to be seen by our eyes, nor perceived in any way at the single cell level (otherwise we'd have an headache if only one cell were undergoing apoptosis?) has always fascinated me. But that Lorenzo, approaching 5 y.o., would ask such a question, was flattering. If you have the answer, please come forward!