Hello Huy It actually does mean "finding the size", because the next sentence is "Professor Binns had asked for a three foot long composition on “The Medieval Assembly of European Wizards." and the next one is "“I don't believe it, I'm still eight inches short....” said Ron furiously"

Hi Huy! I think it means 'considering' or 'thinking about' his History of Magic homework. So maybe he is thinking about if he will do it or not or maybe he has to think about a part of the homework, for example, a difficult question. Hope that helps! Please ask if you have any questions! Mark

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