As cells get larger, volume increases more rapidly than surface area. Since cells exchange materials and molecules across their membranes, a large cell has more difficulty transporting enough materials.
Most single-celled organisms rely on passive diffusion to transport materials across their cell membranes. Many such materials are critically important to the functions of the cell and are therefore needed in great amounts. When a cell has much greater volume than surface area, diffusion is insufficient for providing these materials. This results in large cells being unable to function nearly as quickly or effectively as smaller cells and places a limit on cell size.