Diameter of a circle is any straight line segment that passes through the centre of the circle and whose endpoints lie on the circle. It can also be defined as the longest chord of the circle
The larger the diameter of your castor the smoother journey your moveable object will have when navigating bumps in the floor. Rolling also becomes easier with a larger diameter castor.
Larger diameter castors are considered more essential on objects that you will be moving on a frequent basis. Only use smaller diameter castors on objects that aren't moved very often and, of course, ensure they have the correct load capacity.