Q:

Are there any ancient descriptions of Aristotle's appearance?

Plato wrote several references to Socrates's appearance.  And Diogenes Laertius reports that Plato was called Plato because he was broad shouldered.  Are there any ancient sources describing Aristotle's appearance?

A:

See Diogenes Laërtius, Lives and Opinions of Eminent Philosophers: V,1: Aristotle. for description. 
But Aristotle lived 384–322 BC and Diogenes ca.180 – 240: 5 Centuries is a lot of time !
In general, portraits and marble statues are not reliable sources; but you can see:Ludovisi's Aristotle: roman copy of an original from the Greek Lysippos: 4th century BC and personal sculptor to Alexander the Great.
See: J.H. Jongkees, On the Portraits of Aristotle and Menander, Mnemosyne (1965).

