This is an imporant question but still not an open question. The cell size is governed by internal and external factors. The internal could include a network of signal transduction cascade while external could include inhibitory factors coming from other cells. There are also biophysical factors that are still to be figured out. A recent work in PLOS OnE journal has shown some light how it happens in yeast. The author used a scaling models to show lengths and shape in yeast. (www.plosone.org