How does the size of a cell relate to the size of the animal or plant?

I'm unsure of exactly what you might be asking. I think one question might be about the difference in cell size between plant cells and animal cells. In general, plant cells are larger than animal cells.  

But, I don't feel that is the actual question being asked. I think that you are asking how cell size relates to the size of organisms in general. For example, do large organisms have large cells? If that is the question, then the answer is no. The size of the cell is not related to the size of the organism. Large organisms are large because they have many cells.  

The size of the cell is determined by a few factors, but the most important factor is the surface area to volume ratio. The surface area is referring to the surface area of the cell membrane, and the volume is the capacity of the cytoplasm. Cells want large amounts of surface area to help with diffusion and osmosis, but in general, a cell needs a small volume to efficiently move items throughout the cell. Since volume increases at a cubic rate and surface area increases at a squared rate, the best surface area to volume ratio is found in small cells. However, if the cell is too small, mitotic divisions struggle to occur properly. The centrioles and spindles simply do not have enough room to work.