Doing a simple conversion, 13 millimeters is equal to 0.51181 inch, about half an inch. If this is compared to real-life objects, this is the same size as a small marble of the kind that kids play with.
The millimeter is a unit used for measuring length in the metric system. A millimeter is equal to 1/1000 of 1 meter. In 1983, the meter was given the definition of "the length of the path travelled by light in vacuum during a time interval of 1/299,792,458 of a second." Using another object for comparison, the width of a human hair is approximately 0.02 to 0.04 millimeters; thus, 13 millimeters is about 325 to 650 pieces of human hair placed side by side.