This nylon tag collar is the same as the metal chain type, but this can be more gentle on hair.   The cord is 3/16" thick, same cord as my nylon leads.  The rings are about 3/4" OD
If you order a collar 10" or under,  I will use a smaller ring, about 5/8" OD.
For length either measure a collar you have that fits, or measure around the widest part of the head, usually over the ears.  Gold (solid brass) rings cost extra.