Cells can be in variety sizes and shapes, they may, for example, looks like a cube or a circle under a electron microscope . Scientists who study cells said a cell could be as big as a tennis ball or so small that you can't even describe how small it is!
Bacteria is actually a small single cell, they have various shapes and sizes. Every bacteria contains a special machinery which they live on other cells or damage them, that's how we get diseases like cancer, HIV, AIDS etc.
Cells are usually measured in micrometers which is one millionth of a meter. The smallest bacteria are about 0.2 micron in diameter. The average cell in our body is about 10 microns in diameter.
Study of living organism is called biology, and in this branch, the study of cells is called cytology!
