They are the size of a credit card/hotel room key card. I'm not sure about an Iphone, but my DN's cell phone did demagnetize hers a couple of times before we figured out what was doing it. Of course any magnets on backpacks or purses will do it too. I keep my in a sleeve of some sort or in a separate pocket in my back pack to keep it safe. Sometimes hotels will give you a little cardboard sleeve with their room key in it and I have saved a few of those for my KTTW. Also some BCP's come in a little sleeve that fits these KTTW perfectly.