"This bus is big."
I think you are saying the most accurate explanation. Here's what I think is accurate: kono basu wa okii desu. (Full sentence & polite) kono basu wa okii da. (Full sentence & casual) okii desu. (Shortened & polite) okii da. (Shortened & casual) okii. (One-word brief & casual answer)
The answer that Duolingo wants has no verb. In this case that's "desu" or "da." Omitting them sounds like baby talk or broken Japanese to me.
I'd really like a native speaker or a very fluent language learner to chime in with what sounds right or wrong to them.