"هٰذِهِ الْغُرْفة كَبيرة."
Translation:This room is big.
You'd need a definite article before the adjective:
الغرفة الكبيرة = the big room
الغرفة كبيرة = The room is big.
Your sentence would be: هذه غرفة كبيرة
The Arabic sentence above, has كبيرة (big), which is an adjective, in the predicative position (i.e. the speaker is telling the fact that this room is big).
In your sentence however, the adjective is attached to the noun (called attributive adjective), and the two (adjective and noun) act together as a predicative.
The difference here relies in the definite article (AL). The sentence in the exercise, uses "this" as means to define the "room" but in your sentence, "room" is not defined altogether, but preceded by an adjective and "a".