I am new to this hobby and am in hunt for some washers for motor mounting screws,please tell me how to understand the size ,say M3 ,M4 etc...
What is the diameter in mm for an M3 washer? 

The ID is 3 mm (actually about 3.2)

Thanks, sanjayrai55

If you are looking to learn rather than for a quick fix, go from herehttps://en.wikipedia.org/wiki/ISO_metric_screw_thread

Thanks @Flyingboxcar, for the link.