cells are small because the size helps to maintain a large surface area to volume ratio, to allow for optimal rate of diffusion
A large SA/Vol ratio is required so that there is sufficient area for diffusion to occur to allow molecules to enter the cell and provide adequate material for the cell.
If cells were much larger than they are, it would take so long for nutrients to diffuse from the surface to the centre that the cell would not function properly. To function best, a cell needs a large surface area and a small volume.