My dog's tag is barely bigger than a penny. It only has his name and 2 phone numbers on it - I know the rules say address too, but he's microchipped so he's got all the details on him. (the tag that says he's chipped is quite small too).
we used to have one of those barrel things on our last dog but the end always seemed to come off
We have a gorgeous little Yorkie / poodle cross called Maggi who's about the length of my forearm. I know she needs to wear a tag with my name & address/phone number on, but given her size I can see it driving her nuts. I thought of a collar with the details on, if I can find one with space for enough letters, but how do other people handle this?