"This restaurant is big."
Translation:이 식당은 큽니다.
I'm guessing you mean using 식당이 instead of 식당은. If so, I imagine the sentence would be valid, but it would stress 'this restaurant'. Maybe in an argument about which restaurant is big that would be the way to go.
I beliege that would just be The restaurant is big. Adding "this" To it implies that this specific restaurant is big. Then again dont take my word this is just an assumption but it makes sense