I will introduce the measure of stem swivel caster.
The bore diameter is the internal size of the bore. This measurement is required in order for the wheel to fit the existing axle assembly properly. While looking at the side of the wheel, measure across the inside of the opening in the center of the wheel where the axle passes through. This tends to be a more precise measurement than the wheel diameter. If you have a micrometer or caliper, that would be the preferred method of measuring. Another method would be to insert drill bits until you found one that matched the size of the bore.
Wheel width is the widest part of the tread material. This measurement is not nearly as critical as the hub length, but is very helpful for locating the same or similar wheels. Generally a tape measure will suffice for this measurement.
The last primary measurement is the hub length. This is the distance between each side of the wheel through the bore. Proper hub length allows the wheel to fit between the caster fork assembly with minimal play.
You will notice Hub Diameter and Tread Width on the images above. These are shown for reference and to reduce confusion. Note that the tread width is actually smaller in many cases than the wheel width.
With these measurements, you will be able find a replacement fixed rubber caster.