Theory:

In addition to the variation in shape the cells also show a variation in size. The size of the cell may vary from a millionth of a metre to as large as a few centimetres. Majority of the cells are microscopic in size and hence invisible to the naked eye. These cells can be seen only with a microscope. A cell generally has a diameter of \(0.01mm\) approximately.
 
Bacteria is the smallest cell among the unicellular organisms. It is \(0.1\)  to  \(0.5μm\)  in size. 
 
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Bacteria
 
The red blood cells are the smallest cells present in the human body.
 
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Structure of RBC
 
The egg of an ostrich is the largest cell. It measures \(170mm ×130mm \) in size.
 
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Ostrich with its eggs
 
The size of the cells have no relation to the size and functioning of the animals and plants.
Example:
The nerve cells are an example for this as whether it is an elephant or rat the nerve cells are long, branched and perform the same function, that of transferring messages.
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Neuron or nerve cell
Reference:
https://upload.wikimedia.org/wikipedia/commons/thumb/c/c5/Prokaryote_cell.svg/512px-Prokaryote_cell.svg.png