Do you have documentation or are you guessing? IF you have *proof,* (like an eBay auction or interview statement from Kate) please post a link. About half a dozen sources consistently show her as a US Size 8, with only *one* source (WikiAnswers) listing her as 10.
I don't *like* to guess from photos (only in person), but as a former shoe salesman, I'd opine that the 8 is accurate. Perhaps @warpedmind will weigh in; he's usually fairly close also.