I wouldn't say it's wrong, but you see, when αυτό is used in the sense of it instead of this in Greek, the pronoun would be redundant. So, the sentence would be just "Είναι ένα ορθογώνιο πιάτο."
αυτός (ο), usually followed by an article = this, necessary in a sentence
αυτός, not followed by an article = he, redundant in a sentence.*
*That happens because each pronoun has its own verb type, so there is no space for confusion, especially in speech, where there is also context. ^.^