explain why it is not possible to get the exact size of a cell during cell estimation

Most cells are too small to be seen with the naked eye (the few exceptions include some egg cells, some of which are up to 13 centimeters in diameter, like the ostrich egg). 

Cells are so small because of the constraints of the surface-area-to-volume ratio. Although cells come in many shapes, the easiest to consider is a sphere. The measurement of the outside of the sphere is its surface area, and the space contained within it is its volume. 

For a cell, the surface area is made up of the cell membrane, and the space within it is the cytoplasm (and the nucleus, in the case of eukaryotic cells). 

The cell membrane is selectively permeable, allowing some substances to pass through it while blocking others. This ensures nutrients, water, and other vital materials can enter the cell, and signals and waste materials can exit it.