I think this is in the right place.
I’ve just received some new postcrossing ID number stickers from Zazzle, a person/artist? I’d not used before.
They are double the size I’d expected. They’ll take up about a quarter of the writing space on the postcard. 
My fault, I suppose. I should have checked the dimensions before ordering them, if such an option existed. Has this happened to anyone else?
They are very nice, though. Just too big.
You should try calling them & see if they will make you another order without charge, sometimes they do that - their customer service is pretty good I’ve found.
Completely unhelpful, but I’d love to know where you got them.
I can understand why you are frustrated. I’ve not been able to find good-sized ones so I’ve started making them myself. They measure 4.5cm x 1.3cm (about 1 3/4in x 1/2in)
I’ve gotten them from different places; I think overall the ones I return to are from UnaPhoto on Etsy - she’s also part of Postcrossing; to clarify, I don’t know her nor do I have any affiliation with her Etsy shop. I’ve gotten some cool ones from Papersisters, but they are bigger and I also prefer to keep the space to write.
I also got a weather stamp from Papersisters and, depending on the card, I might just use the stamp and write the ID on a different, bright colour (red, orange, bright blue, etc). Some cards have an explanation of the image and I prefer not to write over it, so it always depends a bit on each case. If the person has their profile almost empty, they get the bigger stickers lol
