Okay, we can say that the reason why cells are so small it's because having those sizes facilitate the metabolizing processes, like the uptake of nutrients, and the outtake of waste products. This brings the cells also a higher speed for these processes.
Being like that, they have better contact with the extracellular media, due to a higher ratio rate.
Regarding the lower bound of the size of a cell, considering that human cells average between 0.1 millimeters (100 um), the smallest cell described is from a bacteria known as Mycoplasma, which has a size of 0.01 millimeters (10 um).