Just bought a nautical hoodie in the whistles sale; I'm normally a 12 but they only had two 10's left so thought I'd try one on. It was a nice fit and quite a loose boxy shape, when I took it to the till the girl said "oh, it's got the wrong tag on it, it's actually a 6" , so she suggested I try on the other one as it was actually a 10. This absolutely swamped me and felt as if I was wearing one of DH's old sweatshirts - the guy working in there said " it's meant to be a loose fit" - but that loose? 
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Anyway I bought the 6 as it fits much better, but have I made a faux pas should it be an incredibly loose fit? I felt completely wrong in the 10 so I guess I did the right thing.