No, it's "Kitaplar büyük" ("the books [are] big"). "The books" is the subject, and not the object, so you can't use the accusative case. However, "Büyük kitapları." is a (grammatically) valid sentence, in which "kitapları" is a direct object. This means it could be something like a reply to a question "Which books did you take from the shelf?" "The big books" (Büyük kitapları) = "[I took] the big books".