"The sun is big."
Translation:सूरज बड़ा है ।
Yes. बड़े is used only when the subject is plural.
Note: When used with people, it can be used even when we are referring to a single person Eg: मेरे पापा बड़े है। but this is because when we are being respectful, the words referring to certain people are treated as if they were plural.