Can you think of an example of something that exists in one size and one size only and is manufactured for one purpose?
I can’t. Yet.
Some suggestions talking to colleagues (and my uber-pedantic reasons why they don’t count):
Red Nose – they make them for cars too
Earrings – I reckon they do different sizes
Make Poverty History rubber wristband – I had a fabric one earlier in the year
Any more for any more?