Here is a handy pet tag size chart.

 

This sizing chart shows the tags, relative to a a UK 50 pence coin.
SCROLL DOWN to see different tag styles. You are also welcome to print it out for reference purposes.

Make sure you have a spare pet tag
Make sure you have a spare pet tag
Make sure you have a spare pet tag
Make sure you have a spare pet tag
Make sure you have a spare pet tag