There are a few reasons why cells are so small. The smaller the cell is the faster it can communicate which means that it will be able to diffuse faster and with much less complications. Also since cells are small, they can go through reproduction smaller. Just think if we had large cells to reproduce? That might take much longer. If the cells are small, they also wont destruct as easy due to the tough cell walls around a smaller cell. If a bigger cell had the same cell wall, i think it would be easier to be damaged. So the main point about our cells being small is that they are all around faster and more efficient.