Cell Phone Wallet
This is a silicone "cell phone wallet". You peel off the paper on the back and it sticks to the back of your cell phone. You can put credit cards, hotel room keys, business cards, or whatever in it. I find that it can hold 4 items comfortably (I carry 3 credit cards and my driver's license in it).
I have an iphone 11, so you can see about what size it is when it's on the phone.
Actual dimensions are 3.5 inches long and 2 inches wide.