Cells need to be small in order to make exchanging materials faster. This is because, as the size of the cell increases (if we were to increase the size), then the volume increases faster than the surface area. We want more surface area, so this isn't effective if we increase the size. The larger the size also means that it takes more energy for them to move, so there will need to be more cytoplasm, and that's too much effort to generate. Imagine being in math class when you use those cubes, the one huge cube assembles 100 small cubes, that one huge cube has less surface area than those 100 small cubes combined. That's why we need small cells. A larger surface area allows for more exchanges to occur and a larger volume allows for more materials to be brought in or released from the cell at a given time. (The rate of exchange would increase for larger surface areas and volumes of cells.)