I can imagine saying
'I am certain about the size.' or 'Are you certain about the size?'; however, I cannot fathom a time when I would speak the sentence 'You are certain about the size.'
So what? The purpose of Duolingo is not to give you a repertoire of sentences, but to show you how words work so that you can speak afterwards.
Ever had a daughter make a fashion faux pas?
I thought is was a question which would have made more sense "are you certain about the size?"
Is this also correct?
You're sure about the size = Tu sei certo SULLA misura
Tu sei certo della misura - means - You are sure about the measure, which is what I wrote and was marked incorrect. You are sure about the size - should be - Sei sicuro della dimensione. DL should stop changing the meanings of words.
Calcola a quale TAGLIA corrispondono le tue misure (Calculate to which SIZE your measurements correspond). DL misses again by forcing a translation via a synonym, and without any context for the proper vocabulary (e.g., Clothing comes in sizes; a crate has dimensions or measurements.)
it rejected my: you are certain of the measurements. should have accepted it
What is the difference? - You are sure about the size. (Correct according to DL) and - are you sure about the size - (Wrong according to DL) - both are questions and mean the same thing!