Answer:
it is necessary for cells to remain small in size because they need to keep a surface area to volume ratio that allows for adequate intake of nutrients and exchanging materials between the inside and outside of cells.
Cells are small so they can have an efficient surface area to volume ratio. This is going to allow the cell to easily excrete waste product.
That is why the cell needs to be small. The smaller it is, the larger the surface area to volume ratio is.