"هٰذِهِ الْغُرْفة كَبيرة."
Translation:This room is big.
This is the big room = هذه هي الغرفة الكبيرة.
Notice that in Arabic, typically the predicative adjective (the adjective that comes after "is" in English to describe the noun before "is"), usually comes as indefinite. Now, to analyze the Arabic sentence, we see: هذه الغرفة كبيرة. First of all, هذه means "this" and it needs something to point at, and here comes الغرفة (the room) which as we can see it is defined with (AL). Thus, our subject is هذه الغرفة here. Where is the predicate of the sentence (the thing that tells us the information we need to know)? Here comes the next word, كبيرة and we notice this adjective is indefinite (no AL attached to it). Thus, كبيرة MUST be the adjective we need AFTER "is" in English. Putting things together:
This Room (is) big (of course we don't say "this the room" in English so "THE" is naturally removed).
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".