What limits the size of a cell?

Cell size is limited by multiple limiting factors. The main reason their size is limited is because of surface to volume ratios. A higher surface area to volume ratio is more efficient than a smaller ratio. The inability of the surface area to increase fast enough (if the cell was too big) would not permit the cell to uptake enough nutrients for its metabolic processes. Cells benefit when they have a high surface area compared to their volume since there is a larger portion of plasma membrane relative to the interior of the cell. This increases the efficiency of the cells to transport substances into and out of cells  as a result. 

An additional limiting factor is the speed at which a protein can move across a membrane. As cells increase in size, the time it takes for proteins to diffuse across them also increases significantly. More specifically, it is known that as the size of a cell increases by a factor of 10, the time needed for proteins to diffuse across the cell increases by a factor of 100. When cells reach 100 micrometers in size, the time taken for proteins to diffuse across the cell can become a limiting factor. This means that while small cells are restricted by the rate at which proteins are synthesized, larger cells may also experience limitations due to the movement of proteins around the cell.