"How big is the universe?"
Translation:Kiel granda estas la universo?
Why is it not "Kiom granda estas la universo?" "Kiel granda" How (in what way) big. "Kiom granda" How (much) big. Where does my reasoning fail?
Kiom accompanies nouns or verbs, not adjectives. You cannot quantify a quality. Instead, you grade it with kiel.
Thanks to a comment of SquirlRat I have noticed that my comment was too categorical. According to PMEG (look under “Kiom kaj tiom pri emfaza grado”) there exist some instances of use of kiom to strongly emphasize degree instead of amount. It also says, though, that in order not to wear out this feature of the language it should be used sparingly.
Duolingo does now accept 'Kiom granda estas la universo?'. If you feel this to be in error maybe it should be reported?
You are right. Thank you for bringing this to my attention. I am editing and correcting my mistake.