I have plenty of 'working in a supermarket' stories but dont think my username offers the anonymity required to post a lot of them
As for coffee shops, they used to intimidate the hell out of me. So many choices, and I'm also not keen on what I viewed as being annoying by asking what everything was. A little research into it though has helped me overcome that fear.
As for the sizes thing; I think its a little weird to only have 2 sizes. I remember reading somewhere (maybe even Howard Shultz book about Starbucks) that if you have small, medium and large, people will generally go for the medium or large, so its perhaps an opportunity to introduce a more profitable medium size. Another word on sizes, it bugs the hell out of me when people ask for small/medium/large in Starbucks. Im not sure why it winds me up so much, but as far as Im concerned those sizes dont exist in there. To me its on the same level as going to Burger King and asking for a Big Mac. Its not on the menu.
Its good to see though that your managers dont take the ridiculous 'customer is always right' line which is clearly not true.