What is cell surface to volume ratio and why is it important in cell functioning? can it explain why cells are so small? .

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. 

Cells are always made small because the ratio of its size should be small to make the reproduction and the regeneration of the plane specially in the organ will be replace cell by cell. If the cells are too large than they usually are it would too much time for them to absorb nutrients and energy that is used for regeneration, replacement and maintaining cell. 

Fungi is were it belongs