Seven and a half? I thought the original Mad Hatter's hat bore a tag that said 10/6, meaning ten shilling sixpence, a sum of money also referred to as a half-guinea. For the alluded subject matter, I would think 10/6 (or 10 1/2) would be a more plausible "size" than 7 1/2 also.
10 1/2, 7 1/2, It was a long time ago. Give me a break here. If I were any good with numbers I wouldn't be working for the government. Anyway, it is the bigger picture that matters.