Most microorganisms are 0.2 micrometer in diameter and 2-8 micrometer in length with a number of shapes, ranging from spheres to rods and spirals

The magnification is different for different bacteria. For proteins you need a resolution of 10 nanometers and for viruses you need a resolution of 100 nanometers. For bacteria, optical microscope resolution should be 1 micrometer and for cell it should be 10 micrometers.