"A big letter" didn't sound correct t me, so I tapped on it to check, and it said "a capital letter", which made more sense to me. So I changed my answer to "a capital letter", and of course got it wrong. The hints they give when you tap on the words shouldn't mislead us.
I had the very same experience. And I was going to type "a big letter"strange though that sounded-- because Duolingo has weird phrases rather often in every language--- until the hints suggested otherwise, and then, like you, was marked wrong. I am suspecting that really that phrase is used to mean capital letter. I notated a problem in the Report section.
Does Lìtir have an accent on the first i? In other answers it says correct when I use the accent, but on this one it says pay attention to the accents when I put in that accent. (Not sure if it's meant to be different here, or my keyboard is somehow always screwing this one up, or if the 'correct' answer has a typo.)