μέτρον τὸ ἡρωικὸν
ὀγκωδέστατον τῶν μέτρων ἐστίν

Aristotle, Poetics

Aristot. Poet. 1459b

τὸ δὲ μέτρον τὸ ἡρωικὸν ἀπὸ τῆς πείρας ἥρμοκεν. εἰ γάρ τις ἐν ἄλλῳ τινὶ μέτρῳ διηγηματικὴν μίμησιν ποιοῖτο ἢ ἐν πολλοῖς, ἀπρεπὲς ἂν φαίνοιτο: τὸ γὰρ ἡρωικὸν στασιμώτατον καὶ [35] ὀγκωδέστατον τῶν μέτρων ἐστίν (διὸ καὶ γλώττας καὶ μεταφορὰς δέχεται μάλιστα: περιττὴ γὰρ καὶ ἡ διηγηματικὴ μίμησις τῶν ἄλλων), τὸ δὲ ἰαμβεῖον καὶ τετράμετρον κινητικὰ καὶ τὸ μὲν ὀρχηστικὸν τὸ δὲ πρακτικόν.

Experience has shown that the heroic hexameter is the right metre. Were anyone to write a narrative poem in any other metre or in several metres, the effect would be wrong. The hexameter is the most sedate and stately of all metres and therefore admits of rare words and metaphors more than others, and narrative poetry is itself elaborate above all others. The iambic and the trochaic tetrameter are lively, the latter suits dancing and the former suits real life.

Aristotle. ed. R. Kassel, Aristotle's Ars Poetica. Oxford, Clarendon Press. 1966.

http://www.perseus.tufts.edu/hopper/text?doc=Perseus:text:1999.01.0056:section%3D1459b